Added redirects to gitit (fixes #233)

You can now create redirect pages by adding the a "redirect" field to the metadata of a page. It works as follows:

 - When a user visits a redirect page, they are redirected to the destination page.

 - At the destination page, a message is displayed telling the user that they have been redirected from the source page. A link is provided back to the source page, with a parameter to disable the redirect (so that it can be edited).

 - Gitit will detects circular redirects and explain to the user what is going on in such cases.