TST: Add numexpr arithmetic tests.

Only add 'div' if not python 3

Also checks dtype in test cases for series
ENH: Allow evaluate to pass kwargs to numexpr
BUG: Fix __truediv__ numexpr error

by passing truediv=True to evaluate for __truediv__
and truediv=False for __div__.
TST: Make test_expressions cover more arithmetic operators
BUG: Remove broken frame mod and floordiv str_rep

`%` (modulus) intermittently causes floating point exceptions when used with
numexpr.

`//` (floordiv) is inconsistent between unaccelerated and accelerated
when dividing by zero (unaccelerated produces 0.0000, accelerated
produces `inf`)
TST: Add informative error msg on null
TST: Add test cases regarding default axes
TST: Remove incorrect alias for div in Python 2

div only does floor division in python 2

whereas everything is truediv in python 3
CLN: Add str_rep to frame __floordiv__
BUG: Fix _fill_zeros in arithmetic methods.

BUG: _fill_zeros needs to be called every time

Previously, _fill_zeros only was called when numexpr's evaluate was
successful. This led to inconsistencies, so now it is called every time.

BUG: Fix div to be consistent and fill zeros
PERF: Use expressions with Series & Panel arithmetic operators

PERF: Make panel use expressions for arithmetic.

panel now uses expressions.evaluate to attempt to speed up
scalar arithmetic operations. Note that, for the moment, `%`
and `//` are not accelerated (`%` b/c of floating point exception
`//` because `numexpr` does not accelerate it).

This includes fill_zero wth all evaluations.

PERF: Make Series use expressions for arithmetic

NOTE: Excludes `%` because of floating point exception issues with
numexpr. `//` is not accelerated because numexpr does not currently
accelerate it.

Series now uses _fill_zeros for all arithmetic evaluations.

Also: made series' method factories set '__name__'

This makes series' factories consistent with the
other arithmetic factories.
TST: Add all arith expressions to frame, readd div

Given fix to div, floordiv, no longer excludes them when testing
flex_frame with integer dataframes.

Also adds cases for other operators + truediv.
BUG: Fix floating pt exception in __mod__

By removing the acceleration (fails on Travis CI right now too).
Will investigate in the future.
ENH: Add 'mod' method to frame

NOTE: This is currently *not* accelerated.
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.
TST: Fix Python 3 operator.div/operator.truediv issue
TST: Add explicit 'other' for arithmetic tests to facilitate testing Panel
TST: test both series and frame flex methods under numexpr