New functionality for adding private RRs to dns lib.
Added docstrings, tests and example for PrivateRRs.

(renamed CustomRR to PrivateRR)
Lots of renamed funcs and structures. More readability.
Changed logic to typeswitch and added comments.

And also added my name to contrib list as promised before.
Merge pull request #131 from asergeyev/customrr

Added Private RR support