Attempting to implement Kernel#sleep

which appears to work, but this is my first foray into RPython goodies.
Sleeping more than once, but for a smaller net time duration

although I'd like to be able to test the zero-arg case.  Not sure how to
do that without using a Thread or similar (?)
Handling the zero-arg case for Kernel#sleep
Adding self to AUTHORS per contribution guidelines :blush:
Revisions per feedback

including less float-casting tomfoolery, asserting allowed argument
counts, and returning something closer to the real amount of time slept
per the described `Kernel#sleep` behavior.
Handling the 0..1 args case with keyword arg instead

and letting someone else raise ArgumentError.
A few more cleanups per feedback

including removal of the argument parsing test since it's not the
business of this test to assert such behavior and disabling the zero-arg
"big sleep" case until we have threads.
Correcting which error type is raised and spiffing up signature variable name.
Meant to test the unimplemented state of zero-arge case.
Merge pull request #346 from modcloth/kernel-sleep

Kernel#sleep