'setup.py test' also uses SystemExit, with args[0] as False.
Detect and re-raise.

Add support for building without extension even when compiler
is functional, and set up extension-less travis builds.

Allow the compilation of the extension to fail, and switch
to pure python mode.

Causes issues if the locale is not set to utf-8

* Removed unittest2 dependency on Python >2.6.
* Moved discover_tests.py into tests/.
* tokenizer.c: Fixed errors noted by -Wshorten-64-to-32.