Implement the ability to store uploads on a per-page basis

Storing all uploaded files in a single directory kinda sucks when you've got
a largish wiki, or the possibility of filename collisions.  With this patch,
though, you can set `:per_page_uploads => true` in your wiki settings and
have the file uploaded to a directory named for the page you were on when
you clicked 'Upload'.

This PR depends on gollum/gollum-lib#49, as there are some small changes to `gollum-lib` needed for this.