Support file input type.

Fixes issue #144, where file input elements are not bound.

Example:
```html
<input bind-files="files" type="file" multiple="multiple"/>
```

```dart
import 'dart:html';
//....
  FileList files;

  void buttonClick() {
    for (File f in files) {
      // do something with f
    }
  }
```
Merge pull request #434 from yohcop/patch-1

Support file input type.