commits method should allow max-count and skip as parameters

[ryanleecode]

The commits method currently tries to pull in all the commits of a repository since inception. This is very infeasible if the repository is very large, like the Linux repository. The methods relies on the underlying git command rev-list, which takes parameters max-count and skip. Implementing these into the library would be beneficial for applications that only want the first 100 commits instead of 700, 000.