Editor file insertion

This pull request allows the user to use relative urls when inserting links and images. (I've already made a pull request for the bootstrap-wysihtml5 library to include the changes made on their library 
http://github.com/jhollingworth/bootstrap-wysihtml5/pull/307)

It also shows existing images on the editor file insertion modal. The user is not allowed to remove nor upload a new one from here, but could be in the future.
 
![file_upload](https://f.cloud.github.com/assets/169639/1351829/54ba96ac-372d-11e3-814e-8df63c904f47.png)