Add bind method to core/common
ENH: Add classmethods to generate arithmetic operators in core/generic
ENH: Use numexpr + normalize signature throughout.

BUG: Fix ``_fill_zeros`` call to work even if TypeError
(previously was inconsistent).
ENH: Abstract arithmetic into core/generic

SparsePanel has to opt-out because it doesn't respond to ``shape``.

ENH: Add flex comparison methods to Series and Panel
DOC: Document entire range of arithmetic methods in api.rst
CLN: Add testing util in core/expressions

Adds a set of testing methods to check that numexpr was actually used
successfully.

Also changes the if hasattr idiom --> getattr.
TST: Flesh out test cases to include all arithmetic ops