Refactor pid selector into a directive

- move pid selector code into its own directive
- exposes `$scope.activeProcess` to the parent controller
- called with `<sl-pid-selector></sl-pid-selector>`
- handle changes with `$scope.$watch('activeProcess', function(process, oldVal){});`

#490

@seanbrookes please review
I was thinking since its a type of menu it should go into menu.less and we can put other re-usable menus in there down the road.