cbabots: Add cmdurl support

Bots now support a "cmdurl" flag.  You may supply a URL from which
commands will occasionally be polled.  The URL should contain a JSON
file as an array of lists:

{
    [
        {
            "id": 1,
            "command": ["argv[0]", "argv[1]", "argv[2]"]
        },
        {
            "id": 2,
            "command": ["cmdname", "firstarg"]
        }
    ]
}

On startup, the list will be read and all commands ignored.  It will
then start polling cmdurl.  If an additional ID is added, that command
will be executed on the bot.
irc-botmanager: Fix bot thread-leak

When a bot is replaced, it will now be explicitly paused before the replacement
happens.  This prevents a bug wherein old bots would continue to run.
cbabots: Pull GavelMaster strings out for easy modification
pollboy: Pull out strings for easier editing

In order to make it easier for other people to modify the bot, any
user-facing strings are now pulled out to the pollboy bot class.