Added rtmps protocol on client connection

[jbreich]

The aim here is to support rtmps protocol with the library (rtmp using a TLS connection). No special modification is needed on the server side because we can already provide a net.Listener which could use TLS or not, but on the client side a net.Dialer is used preventing from using TLS.
So I added a new method TLSDial to support this case even if it not perfect. I could have re-used the already existing Dial method to support both rtmp and rtmps but the problem is that I need to provide an extra configuration tls.Config to make it work. Another possibility would be to put this config directly in the rtmp.ConnConfig but the problem is that it is used for both the client & server connection, and it would only be necessary for a client connection. I can update my PR if you think it is a better approach or if you see another one ?

I also removed the function makeValidAddr because it is not used but I can put it back if you think it is needed

[yutopp]

Sorry for my late response...

LGTM. Thank you for your contribution!