Fix a few typos in documentation

Add a Gitter chat badge to README.rst

Remove obsolete code

They haven't been used in a long time.

2.6 is unsupported. 2.6 is long unsupported.

Consistently spell the version checks that remain as PY2 or PY3 to make
them slightly easier to find.

Enable using coverage and coveralls for coverage reporting.

Set this up on travis.

This was just slightly tricky because of the use of 'j99' in the call to
bin/test.

Add a section on the pitfalls of __eq__/__hash__. Fixes #106.

The reader doesn't need to see the class that doesn't define eq and hash.

- Move doctest example into comment.
- Merge the (advanced) sections.
- Typos.

Noteworthy non-included storages: split base storages from others, NEO

Documentation on transactions and threading.

Conflicts:
	CHANGES.rst

Clear Connection.transaction_manager on close. Fixes #114

And other editorial changes.

More changes to come.

This adds documentatuon on transactions and threading concerns.

There are some topics, like application design and conflict resolution
that might want a deeper treatment, but would probably be better
handled through articles or dedicated topics. (I want to avoid
individual topics being too long or weedy to read, where practical.)

Writing this, I stumbled a bit over thread-local transaction managers.
For most applications, they don't add anything over accessing
transaction managers on connections and actually provide an
opportunity to fail. I'm convinced that it should be possible to do
most transaction management through connections and that the API
provided by transaction managers and the transaction package should be
reserved for distributed transactions.

I didn't mention gevent. I thik there should be a section on gevent,
but I think it should be written by someone who's used gevent with
ZODB. :)

Maybe there should also be a section or mention of using asyncio with
ZODB, pr maybe later.

Closes zopefoundation/zodbdocs#13
Closes zopefoundation/zodbdocs#16