MAINT: Make TALib zipline_wrapper a module level function.

Prepare for making the zipline_wrapper operate on multiple sids,
as the needed nested logic will get cramped within the nested function.

Also, should help clearly define the inputs of the zipline_wrapper
function that are needed before it is passed to the BatchTransform
constructor.
ENH: Enable TALib transforms to perform on multiple stocks.

The TALib transform only supported operating on the first value
of a given batch transform panel row.

Instead of returning the one value, even if an panel with multiple
sids was provided, return a dictionary that maps stock to TALib
result.