adding nav service

@holtbp @rantler 

Not sure if there's a better way to do this, but having a `nav` service seems like it would solve the problem of redefining the same navigation functions.

Any controller/view that has an `ng-click` will need to inject the nav service and then add it to the `$scope` like `$scope.nav = nav;`. Maybe there are convenience methods of `$scope` that I'm not aware of?

Also, any thoughts on a consistent pattern for the nav methods? What I have is okay but not great.