Room participation: sendMessage

[chrisabruce]

Send a message event to the given room.

This endpoint is used to send a message event to a room. Message events
allow access to historical events and pagination, making them suited
for "once-off" activity in a room.

The body of the request should be the content object of the event; the
fields in this object will vary depending on the type of event. See
Room Events_ for the m. event specification.