Merge pull request #223 from arthurschreiber/blob_diff

Add Blob#diff.
bump libgit2 to ffb762
bump libgit2 to 84ba49
bump libgit2 to 514485
Start adding merge functionality.
Fix an issue with passing no options to `Tree#merge`.
Add tests for `Tree#merge`.
Add and enable merge options.
Document merge options.
Add Index#each_conflict.