Add public API (erlmqtt) and rename other src files

The public API is a thin veneer on the connection module, with a few
conveniences, conventions, and free (eventually) of records.

In line with the public API being 'erlmqtt', and the application
always having been 'erlqtt', the source files are now prefixed
'erlmqtt_'.
Add a procedure for receiving messages

This differs from the other procedures in that it does not involve
(directly) a connection. Instead, it is for invoking from the consumer
process, which is (by default) the process that opened the
connection. Messages arriving at the connection are put in the
consuming process's mailbox.
Specs for the public API
Update README with public API and more detail