ENH: Add truediv, floordiv, pow & mod to Series

Note that floordiv and mod will not be accelerated right now.
ENH: Add truediv and floordiv methods to panel
DOC: Add new arithmetic methods to docs.
DELETEME: Added verbose messages to expressions
TST: Fix Python 3 operator.div/operator.truediv issue
CLN: Refactor arithmetic methods into a common decorator

BUG: (fixup) There is no 'operator.mull'

Rename arith_factory --> arith_method, shorten docstring

FIXUP: change other part of arith_factory --> arith_method

Only add if method not already in class' __dict__
have add_special_arithmetic_methods add comp_methods too
Frame: Refactor frame flex methods to common func

Rename flex_factory --> flex_method in common

Add flex method - only add if name not in class' __dict__
Frame: refactor rich comparison to common
Series: refactor rich comparisons to common
Panel: Refactor panel arithmetic methods into common
FIXUP: Make method binding Py2/3 compatible
Add subtract and multiply methods to match Panel
Add flex comparison methods and rename flex_method to flex_arith_method
Frame: Refactor flex rich comparisons to common
Add note with question on r* flex methods
Arith: Move 'mod' + '__mod__' to common
DELETEME: better warning message about overwriting
ENH: Make series' method factories set '__name__'

This makes series' factories consistent with the
other arithmetic factories.
TST: Add explicit 'other' for arithmetic tests to facilitate testing Panel