Add quick tests for domain name and TXT rr
Update domain name and TXT string escaping behaviour

Changes to domain name packing and unpacking:
* Escape dot, backslash, brackets, double-quote, semi-colon and space
* Tab, line feed and carriage return become \t, \n and \r

Changes to TXT string packing and unpacking:
* Escape backslash and double-quote
* Tab, line feed and carriage return become \t, \n and \r
* Other unprintables to \DDD

Changes to TXT string printing:
* No longer use strconv.QuoteToASCII
* Instead, pack and unpack string (without length restriction) and
  surround in double-quotes. This isn't the behaviour discussed with
  @miekg - need to come back to this.
Don't export TXT packing functions
Minimise escaping off the wire and in Stringers