C2sstream split

```
...what worked for me was opening a tls(node core module) connection and working through there, but now im stuck in the SASL part that i guess is well implemented in the node-xmpp, i traced the code to find C2SStream and it inherits from Connection but has the tls implementation, im wondering why is it that C2SStream is not exposed in node-xmpp as it is the C2SServer...```

This solves that issue by exposing the class in `node-xmpp.js`.

Additionally I've split `C2SStream` and `C2SServer` into their own files.