Sign in/Sign out functionality

Changes made:
1. Tastypie now understands /api/user/username instead of /api/user/id
2. cookie_util.js created to allow us to deal with cookies
3. csrf_token added to header if user is signed in
4. Populate ItemMan.current_user with a User model of the current_user if signed in
5. Sign out functionality is there, just needs to be moved to the nav
bar later on since it's currently in the meta section of the sign in
form.

@jnwng, @wesvetter