Version 0.10.0 / Sinatra 1.4 Support (WIP)

Howdy!

This pull request isn't ready for merging just yet (still a failing test) related to XSL vs. XML MIME type detection... I wanted to get some feedback on solutions to the issue, though. Check [my comment on rack/rack](https://github.com/rack/rack/commit/627e482f0bd71a5b89b9a24413c0ab1353525f46#commitcomment-3034098) for more context. Any ideas?