add GetDiff Handler
URL for GetDiff Handler
add GetDiff Handler
fixed merge problem
Merge branch 'master' of github.com:joaopaulovieira/gandalf

Conflicts:
	api/handler.go
	api/handler_test.go
	webserver/main.go
Fix Travis CI build for forks

Move package to expected place.
fix URL GetDiff
Merge pull request #137 from joaopaulovieira/master

Create GetDiff Handler and unit tests for him
Merge pull request #135 from scorphus/travis-fork-build

Fix Travis CI build for forks
Refactoring GetForEachRef

Changes:

* Added Ref, GitUser and Link struct
* Added CreatedAt attribute
API doc: Improving GetBranch

Added example result section
API doc: Added GetTag section (Fix  #139)