I18n pages

This is my stab at #816.

Some things to note/I was unsure of:

* I split up `Store#extensionless_path` into a bunch of methods for me to reuse some in the I18n module. Let me know if that's okay.
* I added a minimum of Cucumber tests
* I use `Module#prepend` to override the Store method, but I'm aware this is a Ruby 2.0 feature. There are a bunch of monkey patches possible for this, what are your conventions?
* I couldn't get all the features to pass on the master tip, and so was the case with my branch. Expected?

Feedback welcome.