Pass options to Grit::Repo#remote_fetch

When I use `Grit##Repo#remote_fetch` on a repo that needs to fetch data that takes more than a few seconds, Grit times out. This pull request allows passing options to `remote_fetch` so that I can turn off timeout, or extend the duration.

Also included are commits in order to run tests with git version 1.8.x and includes dependencies in Gemfile and gemspec.