Add an interface to fetch diff btw 2 commits and specify context lines

/repos/<repo_key>/compare/<old_branch_or_tag_or_sha>...<new_branch_or_tag_or_sha>.diff[?context=n]

Refs #49
Fixes #73
add reference output from https://github.com/hulu/restfulgit/compare/initial...c04112733fe2db2cb2f179fca1a19365cf15fef5.diff
seems that the content after the @@ in a unified diff is optional & impl-dependent?
add reference output for testcase for ?context=N param of /compare/, based on our own output
add tests for /compare/x...y.diff API