Add (*Conn).WriteText(message string)

I've made this shorthand for writing text. I find it very useful for "plain-text" protocols/communication and I believe it to be a good addition for the package. 

I'm also thinking about the Read equivalent.
For instance `msgString, err = conn.ReadText()` where a non-TextMessage read will result in an error.
It's a bit more aggressive because of the required type, but then, users require the type anyway..
Maybe add an ErrWrongMessageType to return when the type for the read message is not TextMessage.