feat(uiSref): add ui-sref-on to expose transition promise

[lordnox]

Fixes #1863

@see #1863

[nateabele]

Looks like one of the tests failed?

[lordnox]

Hm.. could it be that the return value of $state.go differs when run on travis or it might be just the karma:past test I missed...
Should it not return a promise? sigh

The only solution I can think of is to wrap the returned value in a $q.when()...
here: https://github.com/raynode/ui-router/blob/master/src/stateDirectives.js#L140

[nateabele]

The only solution I can think of is to wrap the returned value in a $q.when()...
here: https://github.com/raynode/ui-router/blob/master/src/stateDirectives.js#L140

Yeah, I think that's basically what the rest of the tests do. You could also check out the $resolve tests, which have a couple wrappers for testing promises. Thanks. Let me know if you need any help.

[nateabele]

Closing in favor of #1986.