[RFC] Refactor media to be framework agnostic

This is a proposal to find a way to share media code between the CMF project and the Vespolina project. Basically, I have extracted the framework specific code out into its own library https://github.com/iampersistent/Media . (this is all untested at this point) This library would be used in the Vespolina project for handling media related to commerce. Media in the Vespolia project can be more than images, it can also be documents and spreadsheets.

Because there are additional needs with the Vespolina project, the media library would need more features. It would become more like a framework agnostic SonataMediaBundle, as far as the scope of the features. I did notice that the CMF Media seems to be intended to be simple, which may clash with the needs of Vespolina.

I can see a few different options.
* we can leave things as they are and not share media code between the two projects
* we can have a full featured media library shared between the projects
* we can have a simple media library for CMF that is used as a base library for Vespolina media library

There may be other options I'm missing and I'm open to hearing them.