Added `percolate(...)` and `percolator(...)` functions -- similarly to pull request 73 -- but also added unit tests, more verbose commenting, and IMO clearer function signatures.
getting rid of extraneous file
Remove percolator() method, and tweak docstrings.

Sitting down to merge this in, I had second thoughts about percolator(). I think georgi0u was right and that people should just use index() instead. Otherwise, we have to add not only this method (which I'd call "put_percolator()") but also a symmetric one to delete percolators. And that's just a lot of extra routines that don't any knowledge except the '_percolator' string. And then what it we want to count percolators or something? So I think I'll leave this out for now. We can always add it later.
Add percolate() to the Sphinx docs, and update the changelog.
Add percolate() support. Fix #95. Fix #73.