Add callbacks on starting, start failure and stopping of supervisors

[essen]

This would most likely be better for Ranch 2 in #111 as we will have a separate process to better manage the listening socket anyway.

Also related to rabbitmq/rabbitmq-server#260.

[essen]

Another callback that would be useful is one that allows acting on a socket immediately after accept. One could for example do this kind of operation: http://erlang.org/pipermail/erlang-questions/2014-April/078776.html

[essen]

Actually would be good if we had a callback for before accept and a callback after accept (and before starting the process).

[essen]

As far as I can tell these will no longer be necessary in Ranch 2.0 since we will get the socket from ranch:handshake and can act on it immediately.

[essen]

And the starting/start failure/stopping can be done without a callback.