Fix broken tweetbot with CNAMEs and provide non-CNAME option

[lusis]

This should resolve ticket #12 in a way that I think provides the most flexibility. It keeps the original behavior but provides an additional env var to disable CNAME (NO_CNAME=true) usage. I've tested this with and without CNAMES for both Tweetbot AND Skitch:

• Skitch without CNAME - http://s3itch.s3.amazonaws.com/Screen_Shot_2012-10-22_at_10.42.06_PM-20121022-225456.jpg
• Skitch with CNAME - http://s3itch.lusis.org/Screen_Shot_2012-10-22_at_10.42.06_PM-20121022-231510.jpg
• Tweetbot without CNAME - http://s3itch.s3.amazonaws.com/tweetbot/1tquEu.jpg
• Tweetbot with CNAME - http://s3itch.lusis.org/tweetbot/1tquzx.jpg