pluck - for mapping itertools.get over sequences

This commit adds the pluck function, inspired by the function of the
same name in underscore.js.  This function will pluck one or more
elements from each item in a sequence.
Fixed pluck doctests & added more benchmark code
Faster implementation of pluck.

Thanks to @eriknw
Tweaking pluck docstring.
More pluck docstring fixes.
Merge pull request #151 from obmarg/pluck

pluck - for mapping itertools.get over sequences