Updated code to handle escapes in TXT RDATA

Added test demonstrating error in TXT parsing.
Fixed lexing process and slightly updated conversion
back to strings.

See #126 for details.
Merge pull request #127 from asergeyev/txtescapezscan

Fixed TXT parsing with escaped chars
Added punycode conversion placeholders and...

internal functions that provide conversion and tests for those
per #120.
Moved things to subdir and adding functions to convert strings.
Fixed panic calls to be similar to others in library
Updated Punycode funcs

* docstrings
* changes to handling invalid punycode names
* updated tests
Added package header.
Changed label splitting and fixed behavior for final period in names.
Replaced rune(0) to new const
Declared variables prior to entering main conversion loops
Prevented crash on empty string to our funcs +...

* changed encodeBytes to encode and decodeBytes to decode
* fixed few comments
Make rr_mk public and call it TypeToRR
make all setTYPE parse function return the same values
Identical signature for all setXXX functions
Create and fill TypeToParserFunc
Factor the switch code in favor of TypeToParserFunc
Dont export TypeToParserFunc
Dont export typeToRR (old rr_mk)
Merge pull request #129 from asergeyev/master

Code for handling unicode/punycode name conversions v2
Small cleanups