Add proper version management

We now have proper sonames in Mac OS X and Linux, proper versioning on
the pkg-config file and proper DLL naming in Windows.

The version of the library is defined exclusively in 'src/git2.h'; the build scripts
read it from there automatically.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Set short message when changing a commit's message

Yes, finally.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

tagging initial release of libgit2