Validation around the placeholder, directive requires one of `placeholder` or `ng-placeholder`.
The `ng-placeholder` watches for changes to the passed in variable so the placeholder can be bound to a scope variable as `placeholder="{{scopevalue}}"` doesn't work.
Fix the CSS in the read me, also includes .has-success, .has-error and .has-warning styles.
Fix bug in ngModel with scopes.
Fixed some bugs around ngModel that allow it to be optional - can use better-placeholders WITHOUT an hg-model value.
Fixes #5
Merge pull request #6 from SimeonC/master

Fixed some bugs from last PR