allow deletion in image form type

[dbu]

follow up of doctrine/DoctrinePHPCRBundle#37 - @rmsint proposed to use a form listener and handling code to handle deleting an image again.

right now the only way to delete an image is by deleting the document containing the image. this follows the idea of http://symfony.com/doc/current/cookbook/doctrine/file_uploads.html - if the image is null (meaning we did not upload a new one) that information is simply ignored.

the proposition is to have a listener to show a checkbox removeImage that is mapped to the document and calls setRemoveImage if checked, having a way to know that the user explicitly wanted to delete the image. rmsint wrote a gist for that: https://gist.github.com/rmsint/ad4d8804e3ae6fae2160

(ported over from doctrine/DoctrinePHPCRBundle#40 and not a show stopper for 1.0)