Added juxt to functoolz

juxt takes a series of functions and returns a function that is the
juxtaposition of those functions.

This can be useful for applying several functions to the same data, for
example when implementing a split-with function or when you need to map
several functions over a list.
Simplified juxt doctest

Also removed the groupby anti-pattern.
juxt functionality improvements

- juxt now supports funcs passed as a list as well as variadic.
- juxt now returns a generator.
Juxt documentation improvements
Fixed tests for juxt.
PEP8 fixes for juxt.
''' -> """ for juxt
Merge pull request #137 from obmarg/juxt

Added juxt to functoolz