Youtube playlists under dev.

Hi, i added a YouTube Playlist feature, what do you think?
Some tests are needed 'cus i'm working on Windows 8 and this sucks so much i only tested on python 2.7.
Sory if i messed something up, i'm relative new to all this.
#### Changes
- On pafy.py 
 - added a function for fetching Pafy videos from an youtube playlist.
- On ytdl
 - added a new parser `-p` so it can see if it is a playlist or an video
 - now the `main()` work with an array of Pafy objects, so it's easier to call a download function against a whole playlist.