Standardize comments style
Cleanup usings
Make build.libgit2sharp.sh executable
Favor Constants.Signature over BaseFixture.DummySignature
Make RelWithDebInfo the default libgit2 build flavor

Signed-off-by: yorah <yoram.harmelin@gmail.com>
Refactor Travis build script

Signed-off-by: yorah <yoram.harmelin@gmail.com>