Add video annotation support

This work is still work in progress, doesn't seem to work for http://acroeng.adobe.com/Test_Files/classic_multimedia//VolvoS40V50-Full.pdf

It does seem like it worked in the beginning, but doesn't seem to work for me.  

Rebasing change from pull request #4376 to get some reviews and pitfalls. 
Nit: only use space *after* the colon, and not before it. This line should read:
```javaScript
'avi': 'video/avi',
```
The same applies on the next two lines as well.