Callbacks ought not to be passed inline to connect_cached.

So mention that fact in the callbacks documentation.
Add the `connect_cached.connected` callback.

It is only called for new connections, right after they connect. This is
because the `connected()` method is not called, for whatever reason.
Note the reason for connect_cached.connected.

It is to do something after a new connection is made. One can use a
`connected` callback, but that's called for both new and reused connections,
which is probably not desirable.
Merge pull request #2 from theory/cb-doctweak

Callbacks ought not to be passed inline to connect_cached.
Added connect_cached.connected Callback PR#3 [David E. Wheeler]

Merge remote-tracking branch 'origin/pr/3'