Add support for compatible release ranges

[ekmartin]

Fixes #37

[ekmartin]

Sorry, had a typo that failed one of the tests - should be good now.

[rbarrois]

Awesome, thanks!

Could you add an entry in the docs? It should go around https://github.com/rbarrois/python-semanticversion/blob/master/docs/reference.rst#version-specifications-the-spec-class, near the note about NPM (sorry, github doesn't allow to view source code of RST files...)

[ekmartin]

Sure, I'll take a look at it tomorrow.

[MinchinWeb]

[ @rbarrois to view the source of RST (and Markdown, etc) files on GitHub, click on the "Raw" button at the top of the file. Then you get this ]

[rbarrois]

[ @MinchinWeb : Yes, but it doesn't allow to point at specific lines :P ]