Prevent attempts to close the channels twice.
Move constants to the top.
Add comments to explain the difference between Dial and NewClient.
Remove fast path for messages containing a raw string.

This path was faster in some cases, but caused unexpected results in many other situations.
Calculate lenghts for messages and prefixes.
Copy & paste error in TestMessage_String().
Always limit messages to 512 characters as per rfc 2812.
Improve documentation.
Update readme.