Event called when a bot telegram is received.
This event is triggered only in response to a call to aw_botgram_send. This event sets the following attributes:
AW_BOTGRAM_FROM
AW_BOTGRAM_FROM_NAME
AW_BOTGRAM_TYPE
AW_BOTGRAM_TEXT
See aw_botgram_send for an example of how to use AW_EVENT_BOTGRAM.