Added support for file inputs with input directive

Note:  I am opening this PR for some discussion before finishing it off, since I have come upon a problem of testability of this feature.

This feature is for adding support for the input directive for inputs with type file.  This is meant to address [#2568](https://github.com/angular/angular.js/issues/2568).  As far as my hands-on testing with this has gone, what I have here works for updating ngModel with the file data, as well as adding support for ngChange callbacks.  However, unit testing these features causes a problem since you cannot manually set the file property of an input with type file due to official specification.

I would like some guidance on how to best approach this issue so this feature can get implemented.