Adds display of OAM short code on media insert

I've added a new field when inserting a media to be able to copy/paste OAM short code directly.

It doesn't seems possible to add a new option in the select tag after "Display attachment settings".

![embed](https://f.cloud.github.com/assets/103900/1599938/f9e16024-5362-11e3-99ca-9bdeeb9bc263.png)