RDATA items changed to be more space efficient. Distinguishes
  several content types and stores the rest as binary array with
  its length stored in th first byte.

Changed RDATA constructor to not allocate space for items.

Functions for comparing RDATAs:
  dnslib_rdata_compare() - just a first draft though.
  dnslib_rdata_compare_binary() - compares two binary RDATA items
    (needs testing)

refs #119