CLN: conform read_clipboard / to_clipboard to new io standards

     removed to io.clipboard (from io.parsers)
Merge pull request #3824 from jreback/clipboard

CLN: conform read_clipboard / to_clipboard to new io standards
TST: Add numexpr arithmetic tests.

Only add 'div' if not python 3

Also checks dtype in test cases for series
TST: Add informative error msg on null
TST: Remove incorrect alias for div in Python 2

div only does floor division in python 2

whereas everything is truediv in python 3
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__.
BUG: Fix frame's __mod__ str_rep error
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.
ENH: Add str_rep to frame __floordiv__
TST: Make test_expressions cover more 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.

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 uses _fill_zeros for all arithmetic evaluations.
ENH: Add truediv and floordiv methods.

Adds accelerated truediv and (non-accelerated because of how numexpr
works) floordiv, also adds back `str_rep` to floordiv for consistency.
BUG: Fix div to be consistent and fill zeros
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