Make it easier to play: add TempFileStore

[Jason Gerard DeRose]

Long ago, before filestore was split out of Dmedia, a FileStore would automatically create a temporary directory if you didn't provide *parentdir*. In the end this was a bad idea for something like FileStore where we need the API to be very explicit, never to make creative guesses about anything.

But my heart was in the right place: you should be able to play with a FileStore with very low friction, without commitment, without risk of damaging production data. So I'm adding a TempFileStore class, inspired by the TempCouch class that has been so useful in usercouch.

I didn't like this because there are lots of scenarios that could cause *parentdir* to be None without the user meaning "please create me a temporary parentdir":

>>> fs = FileStore(None)

But this I do like, because it's unambiguous:

>>> fs = TempFileStore()

Also, because there isn't ambiguity, it's safe to automatically delete the temporary *parentdir* when the TempFileStore is garbage collected. Which is good for play, I think... by definition it will be throw-away.