Fixing issue with nested dir mixins using incorrect object key

Addresses an issue with mixins defined in a nested directory using bad keys.

For a template `foo/bar.jade` to have a mixin defined:

Before: `exports["foo.bar"]["mixin"] = function(){}`
After: `exports["foo"]["bar"]["mixin"] = function(){}`

Sorry theres no test for this, not sure how you guys are running tests... nudge nudge.