Add The Reaper for freeing garbage collected spotify pointers (see #23)
Do not forget to send subscribers to the reaper
Make note of importance of lack of locks in Reaper#mark
Log exceptions thrown in Reaper#mark
Disable the Reaper during most test runs

For most tests, the Reaper will never run, the tests simply run too
little time for it to be meaningful. We'll write tests for the Reaper
specifically.
Make the Reaper public API even smaller
Add a spec outline for the Reaper
Add specs for the Reaper!
Make sure to terminate the Reaper in before every spec