Refactor how content type is determined.
Remove confusing/complicated loading of creds from s3cfg. Switch to standard AWS env vars.
Actually use tiletype correctly.
Refactor all the tests.
Updated creds.
Merge pull request #19 from mapbox/refactors

0.13.0 changes
0.13.0
linux [publish binary]
Merge branch 'master' into travis-osx