Use libgit2 with threads

I compiled libgit2 with thread support and got no crashes so far. I think it could be the time to enable in in our releases.
@csware 
Is [DllCanUnloadNow](http://msdn.microsoft.com/en-us//library/windows/desktop/ms690368(v=vs.85).aspx) a possible way?

@ch3cooli 
I see your implement in ```ShellExt.cpp```, another possible way?

Thank you. :)