added functionality to save default video speed in chrome sync storage

Added the functionality to allow user to save Default Video Speed via Chrome Extensions' Browser Action popup.

![video playback speed controller](https://cloud.githubusercontent.com/assets/307583/3195058/9c74dbfc-ed07-11e3-9c4a-c2593aa651d8.png)

* Default Video Speed is saved in Chrome Sync Storage
* Change of Default Video Speed via Browser Action Popup updates currently viewed video speed automatically.
* Though changing the video speed from the original video speed controller (positioned at the top left of the video), does not update the Default Video Speed stored in Chrome Storage.