Closes #2587.

Simplify encoding detection and source file opening by starting off reading bytes, and only switching to decoding once we know which encoding to use.

Print run time stats in the test runner

Split C/C++ tests in travis again, but manually this time since many of the build matrix configs are special anyway.

Added the pep8 rule: W291 trailing whitespaces.

Reorder and clean up travis build config to make the most interesting jobs run first: Py2.7/3.7/2.6.

Enable coverage reporting in travis runs. Merge C/C++ jobs into one to simplify that (we have enough jobs to keep travis busy).

Add a "CYTHON_HEX_VERSION" macro in the style of PY_HEX_VERSION that user code can use to detect the Cython version that generated the main module.

Add test for Cython module usage in subinterpreters. In Py3.5, we should now raise an exception when trying to re-import a module into a different subinterpreter.