authenticate users filter and start polymorphic comments

Ok! So, comments are now polymorphic!  Theoretically they should work, but I haven't gotten to testing this current code in the UI or console yet (controller logic may be slightly flawed still).  

Also, I set up an authenticate_user method so that actions which require the presence of a user_id can be protected against throwing an error with a before filter.

Currently, though there is something wrong with the sign up form or authentication, because I am unable to sign up.... I've run out of time to spend on this for the day and need someone else to investigate.