Keep only chat messages with nick mentioned

Before this commit, if Pidgin sends any flag among those at the following url (except for the first 2), no notification will be triggered. For example, if you receive a chat message that contains your nick, the flag will be `0x0002 + 0x0020`. This means that equality is not the way to check for flags. This commit makes use of the bitwise-and operator to check if a particular flag is included.