Build libgit2 as a dependency, for now requiring CMake
Add libgit2 to the default build
Huge commit (not yet stable)
Merge branch 'gsoc-libgit2' of github.com:andrioni/julia into gsoc-libgit2

Conflicts:
	deps/Makefile
	deps/Versions.make
Minor cleanups
Codebase stable, more basic operations done by libgit2

In particular, init, add and update are mostly done