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.

Wasn't entirely sure whether to submit this function to toolz, since it's not
too hard to build this manually using map or a comprehension. I do find
myself doing this fairly often though, so a shortcut made sense to me.
Happy for it to be rejected if it's not suitable.

I've also added a simple benchmark that I used to test the implementation.
I also tried implementing by mapping over partial or curried.get, but both 
were slower than the current implementation.