Issue #26 Use pure python on compilation failure

'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.

Another thing missed in #91

Target documentation for Python 3 usage

2 is dead, long live 3. Mainly turning print info a function
and urllib import fixes

Update README.rst for Pywikibot core

The new version ('core') is much more used and developed
than the previous one ('compat').