Add extractor for Patreon

This will likely eventually need revisions, as it's probably possible for
Patreon to support more than one attachment per creation URL. I'd need to see
an example of this, however, to properly code for it.

```
% python test/test_download.py -v TestDownload.test_Patreon
test_Patreon (__main__.TestDownload) ... [patreon] 743933: Downloading webpage
[info] Writing video description metadata as JSON to: 743933.info.json
[debug] Invoking downloader on u'http://www.patreon.com/file?s=87145&h=743933&i=18405'
[download] Destination: 743933.mp3
[download] 100% of 10.00KiB in 00:00
ok

----------------------------------------------------------------------
Ran 1 test in 1.380s

OK
```