Convert to formula compatible with brew tap; fix OSX for 10.9

As noted in the updated readme, this will allow users to install this formula using `brew tap`.

**For this to work, this repository must be renamed to _homebrew-encfs_.** The existing URLs will redirect to the new repository name and continue to work.

This also includes the updated formula for OSX 10.9 Mavericks noted in #7. This fixes the missing boost symbol issue and closes #7.