Add Repo.openIndex
Added a backing unit test
Merge pull request #198 from nodegit/open-index

Add Repo.openIndex
Support latest libgit2 v0.21.0
Merge pull request #200 from nodegit/latest-libgit2

Support latest libgit2 v0.21.0
Add git_attr methods and ensure prototype exists
Added tests for git_attr