Add FLOAT_CMP to doctest that was failing in Python 2.6, and make yet another tweak to the FLOAT_CMP output checker as to where in an output string it can expect to find a floating point number (I'm really surprised '=' wasn't already one of the delmiters for where a float could begin)
Move the 'mapping' models to their own mappings module, per the suggestion here: https://github.com/astropy/astropy/pull/3231#issuecomment-67657698  This also made a slight tweak to how the special method __call__ is handled in _ModelMeta, so that it is unnecessary for these models, that have undefined inputs/outputs until they are instantiated, to define dummy __call__ methods in order to work.
Fix division on Python 2 in modules where from __future__ import division is *not* use.  This simply requires the __div__ special method to be defined, but only in Python 2.  In this case __div__ does not come with the ambiguities it has for Python numeric types, as there is only one definition of division for models.  (__floordiv__ is deliberately left unsupported).