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
can re move this out into common or ui module and add the module name to the name:

eg: sl[Common]PidSelector

That convention helps when you are trying to find the directive definition from the code