create GetDiff
Merge branch 'master' of github.com:joaopaulovieira/gandalf

Conflicts:
	repository/mocks.go
Merge branch 'master' of https://github.com/tsuru/gandalf

Conflicts:
	repository/mocks.go
refactored CreateCommitOnTestRepository to GetLastHashCommit and unit test
Merge pull request #131 from joaopaulovieira/master

Create GetDiff for return diff between commits
go fmt ./...
updated CONTRIBUTORS.
Added handler to return a list of repository tags (Fix #120)