Allow channel names that start with a '&' character

BitlBee, an IRC to other chat networks gateway, uses "&bitlbee" as its control channel. According to RFC 1459 channels starting with a '&' character are valid channels that are local to an IRC server. This patch changes Convos so that channel names with '&' are accepted in addition to channel names that begin with '#'.

There are two places I'm not sure about:

1. _parse_channels() in Convos/Core.pm uses 'grep { $_ ne '#' ... }'. I replaced that code by 'grep { $_ !~ /^[#&]$/ ... }' but I don't know why that comparison is there.

2. url_for() in jquery.helpers.js might need to escape '&' in addition to '#':
return args.join('/').replace(/#/g, '%23').replace(/&/g, '%26');