Add str.lower() for channel name to set_mask() and show_mask()

topic() calls lower() on the channel name, but not on set_mask() and show_mask(). If the channel name is not lower case, topic doesn't find the topic mask row in the database as the where condition is case sensitive. According to RFC2818 http://tools.ietf.org/html/rfc2812#section-1.3 channel names should be case insensitive, so lower() on all the channel names seems more appropriate than removing lower() from topic()