Change 'this' with $state in $state.go

The change allows to use partial application. Like with underscore's ```partial``` method.
Otherwise, when trying:
```
item.$remove().then(_.partial($state.go, '.', {}, { reload: true }));
```
the ```this``` inside '$state.go' was referring to the global object (window).
(I could have used ```$state.go.bind($state, ...)``` instead, but I wasn't intending to actually 'bind' anything.)

It would also make ```$state.go``` more consistent with the style used in ```$state.reload``` which calls ```transitionTo``` using ```$state``` instead of ```this```.