Added libgit2 features to Repository.Version

Introduced NativeMethods.Capabilities .

Added CanCreateValidVersionString() test .

Resolves #676
Displaced NativeMethods CompiledFeatures to Proxy

Resolves #676
Merge branch 'issue_680' of https://github.com/someoneigna/libgit2sharp into issue_680
Fixed typo in [Obsolete] tag.