automatic versioning

This tries to unify all the different places we currently have Xbmc's version by:
- creating a new file version.txt that holds all versioning related variables
- parsing that file in configure and generating the various scripts for packaging and addons
- replacing GitRevision.cpp with a new class CCompileInfo that gets autogenerated on make(where supported) or on bootstrap / windows build deps step

note: win32 build is still broken with unresolved externals, @wsoltys was already looking into it
thanks to @t-nelson for the guidance
This return can be removed