Adding masquerade support

This required a few things, namely the ability to show a list of users
and then masquerade as them. Thusfar this does not include logging out
once you've masqueraded.

Go to /admin/accounts/stop_masquerading to stop masquerading
Go to /admin/accounts to search for a user to masquerade as
Merge pull request #62 from ndlib/adding-ability-to-masquerade

Adding masquerade support