Added DomainRecord and Record classes to the codebase

[junekomeiji]

I added the DomainRecord and Record classes in the codebase alongside unit tests for them to allow easy adding and removing of records assigned to a particular namecoin domain. These new record classes means that it is easy to extend the records to accept new data types, as well as reducing the amount of redundant code needed to write verifiers for each particular data type.

[JeremyRand]

Thanks Rose! I'll review this in the next couple days.

[JeremyRand]

Per discussion in the #namecoin-dev meeting last month, this PR is on hold to avoid colliding with ongoing collaboration with Q and IETF.