Breakout the mercury requires from others

Having the dependency requires in with the mercury component requires (and indeed the core mercury code) makes life difficult for people wishing to customise there mercury install (Mainly due to the usage of require_tree which makes it impossible to simply copy and paste the file and then override the bits specifically).

By separating out the core mercury requires into another file, people who need to override dependencies can do so, but without impacting the existing mercury workflow. Ideally I'd separate the core code from the requires entirely but I wondered if there was a stylistic or otherwise reason for having it in the same file.

Specifically I've encountered issues doing this the old way with your usage of the custom jQuery ui component. I want to use jQuery from jquery-rails and jQuery UI from jquery-ui-rails which allow me to use the accordion element. The version bundled doesn't play nicely with this setup.