Better handle locale in request.path - solve #268

[Kulgar]

I modified the code a bit to better handle locale (fr, en) in request path, see #268.

I also removed the '/' that was automatically added at the beginning of a slug for two reasons:

• First, I don't see why is that necessary, I think this was because of the use of "request.path" which always starts with a '/'. As my pull request doesn't user request.path anymore, adding the '/' isn't necessary anymore - I think.
• Second, it adds the '/' only for the default locale. Slug is a translatable field, and the '/' isn't automatically added for other languages, so that is a little bit weird.

To keep a backward compatibility, I added [path, "/#{path]"] in Spree::Page find methods. I updated rspecs accordingly and they are all passing.

I wondered if I should also add a unit test for locale in path? You tell me.

Another question: why the gem isn't using friendly_id just like spree_core for products? Any particular reasons?