add starttls support

StartTLS support introduced, so lib can be used to communicate with
servers that wants to `<starttls/>` before real TLS session begins.

`StartTLS = true` and `NoTLS = false` options should be both explicitly
specified to enable delayed TLS session to begin.

Also, added a way to specify nickname when joining MUC.

Also, added 'NoVerifyTLSHost' option to skip cert check for broken
servers.

Possible fix for: #10.