Fix submodule

In my tests, doing `npm install` doesn't clone gitteh, so running `git submodule init` during the install process fails. This patch detects that condition and downloads a tarball of the libgit2 source if necessary. The approach is inspired by [nodegit's install script](https://github.com/tbranyen/nodegit/blob/master/install.js#L35-L47).

This is based on #48, so it probably shouldn't be merged before that one.

- [x] Download a tarball if necessary
- [ ] Rebase if there are changes to #48