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.
another passing spec
Correcting which error type is raised and spiffing up signature variable name.
Meant to test the unimplemented state of zero-arge case.
we pass these specs
Merge pull request #346 from modcloth/kernel-sleep

Kernel#sleep
small nits