Add mapply and associated tests
Add remove-keys and remove-vals, update docstrings.

Create the remove-keys and remove-vals functions as opposites of
 filter-keys and filter-vals.  The docstrings for all four functions are
 worded to be consistent with clojure.core's filter and remove functions.
Merge pull request #6 from ToBeReplaced/mapply-remove-keys

Add mapply, remove-keys, and remove-vals.