Fix priority colors when using colorblocks.

Removed `topydo.lib.Colors.Colors.get_priority_colors()` and added in
its place `topydo.lib.Colors.Colors.get_priority_color(p_priority)`.

Also moved `_name_to_ansi`, `_name_to_int`, and `_get_ansi` outside of
the class.

Conflicts:
	setup.py
	topydo/lib/PrettyPrinterFilter.py

Or: I should just run my tests first.

Clear ANSI codes before output length calculations

This fixes bug of truncating too much text if `%S` and `%Z` were used in
list_format at the same time.

The Todo class is now able to calculate this when no start date is given.

An explicit start date is given with the t: tag (unless configured
otherwise). Together with the due date, the start date is used to
calculate the length of a todo item (in days).

When no explicit start date is given, consider the creation date as the
start date (if present).

When no recurrence pattern is given, nor an explicit start date with t:,
use the creation date as the start date to determine the length of a
todo item.

A strict recurrence is prefixed with +. Simply prepending a minus sign
results in an invalid pattern -+5d. It should be +-5d. But strict
recurrence is not of importance when calculating the todo's length, so
discard the + altogether and replace it with a - to get -5d.

All sensible one-letter abbreviations are already taken (c for color, p for progress, h for hue, ...)

A color block is basically a space with a background color and can be
inserted with a special placeholder for list formatting (not in this
commit). The color is determined by attributes of the todo. This
implementation focusses on the due date.

A color is assigned on a range from green to yellow to red, where red is
overdue, yellow is nearly overdue and green is still 'safe'.

This is relative to the length of the todo item (due minus start date).
The start date can be determined in multiple ways:

* an explicit start date with the t: tag
* the due date minus the recurrence period (rec: tag)

The recurrence takes precedence over the explicit start date in
calculating the length. The primary reason is that t: is mainly used for
filtering purposes.

This is still experimental, I'm just pushing my work in progress here.

TODO:

(B) Add tests
(C) Determine color based on other attributes such as importance,
    priority or a value of a user-specified tag
(C) Revisit the Colors class to fit nicely in the design

Improves branch coverage. Also related to commit cde8f8ed.

pypy was still broken in the Graph class, but in this branch this is no longer an issue.

The lambdas, which are not all run, cause coverage to think that the
jump to the function exit was never made, polluting the results.
Therefore add this pragma to mask these instances of missing branch
coverage.

See also:

https://bitbucket.org/ned/coveragepy/issues/90/lambda-expression-confuses-branch

The result of a minor merge error.

This reverts commit bbdfb2a5.

(Wheel) dependencies are evaluated at build time, so they should stay in
extras_require, which are evaluated at install-time.

See issue #79.

Add new tests to increase lib coverage

Mainly adds a missed case in ExpressionCommand, where priority filters were not covered.

Supporting Python 2 resulted in some additional dependencies (six,
ushlex, get_terminal_size). Removing 2.7 support makes things a bit
easier and maintainable.

Installing from the source distribution (sdist), ushlex and
shutil_get_terminal_size were not installed. Putting the condition in
install_requires addresses this.

Oops! Almost forgotten.