Login, Constraints, Oh My!

Added a login screen. Checks if the user is already logged in (cached)
— if so, go straight into the Home screen. Else, keeps at the Login
screen.

If the user logs out (from Profile), then the user is logged out
(PFUser.logOut()) and returned to the Login main screen. Do not unlink
from Facebook.

Added constraints to the Messages view controller. This also supports
dynamic cell heights for the messages. We’ve also refactored the code
to utilize a peoplePics dictionary for the messages’ profile pictures.
This should be a bit quicker than using a PFUser query all the time to
find the picture.

We’ve also cleaned up the constraints for the
ListingDetailViewController. Essentially, we’ll increase the
ScrollView’s content CGSize if, and only if, the listingHeadlineLabel
and/ or listingDescriptionTextView increases. We calculate the changes
in dimension to those two objects and add the difference to the
listingScrollView dimension.