Adding picture feature to Dr. profile

* Using paperclip gem to handle uploading images
* Generates migration to add picture column to user table
* Adds paperclip gem to Gemfile
* Adds picture field to edit form
* Adds picture display to user show view
* Implements styling options for paperclip (medium and thumb)
* Adds path in development environment for paperclip to use
* Makes user authorization for update and delete clearer