Remove Dependency on Angular's debugInfo; Allow for Data Attrs

This is intended to resolve #291. The directive has access to a `scope` object capable of running `$eval`. Using that instead of `droptarget.scope()` frees up the dependency on the debugInfo allowing this library to run in environments that turn the debugInfo off.

In the issue, I misread the `attr` to be the Angular `attrs` (which is agnostic to whether or not you use a data attribute). To better support both possible sets of users, I check for ng-model first and then try to assign based on `data-ng-model` if that fails.

Coveralls will provide further insight, but the karma tests (at least when run in PhantomJS) continued to pass.
I like this a lot, nice catch!
Can you make a separate PR with this but without the `if`, since I think it is handled by the `if (ngModel)` in line 66.