weeprowl version 0.5.

Closes #4 - Weeprowl blocks execution of Weechat when submitting
notification: weeprowl now makes Prowl API requests via WeeChat's
`hook_process_hashtable` mechanism instead of using Python's `httplib`.
This allows requests to be backgrounded without blocking execution of
WeeChat.

Closes #5 - Allow the ability to set a global prowl priority level, or a
per-type level: Added `prowl_priority` setting.

Minor refactoring and stylistic changes.

weeprowl now requires WeeChat version 0.3.7 or greater.