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'.
Merge remote-tracking branch 'upstream/master' into per-page-uploads

Conflicts:
	lib/gollum/public/gollum/javascript/gollum.js
Merge pull request #781 from anchor/per-page-uploads

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