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
this is pretty generic - it's not very clear what this is actually for.

Can we make the names a little more specific to host/pid filtering?