Youtube API v3

Update to Youtube API v3 as the v2 API will stop working on April 20,
2015.

This change introduces new functions to:
* Get the channel ID based on the Youtube username
* Get the upload playlist ID based on the channel ID
* Save both to the user meta
* Get the latest uploads based on the playlist ID
* The results will be saved in a transient for 24 hours

Other changes:
* Use the native WordPress video player for 3.9 and later
* Simplify the code
* Better inline documentation
* Bug fixes
Update the options page

Removes the width and height options. Will use the default embed sizes.
Update readme.txt