Add TypeToTypeString and ClassToClassString

After some discussion with Robert Edmonds, these helper functions
seems in order. It is not what he requested (that was an RRType
and RRClass as new types), but is does make sense to add some
convience functions to save some typing. The dns package itself
also used these function (4 times). So clearly there is a need.

I'm not completely sold about the names.
Created two new types: Type and Class

Actually just go with Robert's suggestion as this seems the
cleanest way to do it. Also saves me from thinking about could
names for the new functions.