adding isDisabled

[sgentile]

I updated the code to support isDisabled.

It properly passes a boolean value to the isDisabled as an optional parameter (=?). Since it's a boolean I changed the attrs.observe to scope.watch

is-disabled="true"

Also, this was using the outdated ngmin so instead I'm using ng-annotate

[cgarvis]

How is this better than the current version?

[cgarvis]

use ng-disabled to set disabled properly. Works in the latest release (1.2)