Don't show suggestions on ng-model pre-population

When ng-model is prepopulated by `$scope`, the suggestions open up. 

This is undesirable since a user may populate the ng-model of `autocomplete` by `$scope.foo=bar` without user intervention on an initial load. This  Since `onfocus` is sketchy here, this is the most efficient way to go.
Merge pull request #23 from ncthis/master

Don't show suggestions on ng-model pre-population