Initial checkin of Poll plugin: micro-app to post mini polls/surveys from the notice...

Initial checkin of Poll plugin: micro-app to post mini polls/surveys from the notice form.

This version is fairly basic; votes do not (yet) show a reply, they just got in the table. No pretty graphs for the results yet, just text.
The ActivityStream output is temporary and probably should be replaced; the current structures for adding custom data aren't really ready yet (especially since we need to cover JSON and Atom formats, probably pretty differently)

Uses similar system as Bookmark for attaching to notices -- saves a custom URI for an alternate action, which we can then pass in and hook back up to our poll object. This can probably do with a little more simplification in the parent MicroAppPlugin class.

Currently adds two tables:
- poll holds the main poll info: id and URI to associate with the notice, then the question and a text blob with the options.
- poll_response records the selections picked by our nice fellows.

Hopefully no off-by-one bugs left in the selection, but I give no guarantees. ;)
Some todo notes in the README and in doc comments.