ng:class as well as ng:class-odd and ng:class-even always reset the
class list to whatever it was before compilation, this makes it
impossible to create another directive which adds its own classes on the
element on which ng:class was applied.

the fix simply removes all classes that were added previously by
ng:class and add classes that the ng:class expression evaluates to.

we can now guarantee that we won't clobber stuff added before or after
compilation as long as all class names are unique.

in order to implement this I had to beef up jqLite#addClass and
jqLite#removeClass to be able to add/remove multiple classes without
creating duplicates.

- both $watch and $on now return a function which when called
  deregisters the listener
- $removeListener was removed and replaced with the above
- added more tests for $watch and $on

Closes #542

- fixed traversal originating on a scope with with a right sibling
- unified code for both $broadcast and $digest

BREAKING CHANGE
* removing `onChange`

FEATURE
* adding three events: $beforeRouteChange, $afterRouteChange, $routeReload