Add BitTorrent Sync

Created a .gitignore file that covers files and folders currently dropped by [BitTorrent's Sync][btsync] app.

A list of these files can be found in the [Official FAQ][ofaq] and the [Unofficial FAQ][unofaq].

[btsync]: http://labs.bittorrent.com/experiments/sync.html
[ofaq]: http://forum.bittorrent.com/topic/16410-bittorrent-sync-faq/
[unofaq]: http://forum.bittorrent.com/topic/17782-bittorrent-sync-faq-unofficial/