Better installation flow for developing.

- Detects if libgit2 is already fetched instead of constantly fetching.
- Provides more meta information about libgit2 inside package.json.
Removed all references to cmake.
Ensure that libgit2 is removed if outdated.
Merge pull request #180 from nodegit/better-flow

Better installation flow for developing.
Removed all generated C++.
Ensure code is generated before attempting to install.
Fixed missing source from copy.cc.