Add modal to add youtube.

#75 
![video_library___site-install](https://cloud.githubusercontent.com/assets/1222368/4364424/f51288e8-429f-11e4-9787-5bd6cfc22aac.png)
![video_library___site-install](https://cloud.githubusercontent.com/assets/1222368/4364441/07f2699c-42a0-11e4-8d19-f7322efb89b9.png)