Fix .btn-file class

This will fix few issues with file upload button styled with .btn-file class, in particular:

- fixed size hardcoded in css
- cursor set as pointer only for part of the button

Taken from Stack Overflow, [here](http://stackoverflow.com/questions/11235206/twitter-bootstrap-form-file-element-upload-button). More info can be seen in the [original author's article](http://www.abeautifulsite.net/whipping-file-inputs-into-shape-with-bootstrap-3/), check the comments.