Modified also the helper functions fill_data() and check_data()
  - they are now storing all what is saved to the RDATA, so it
    can be used later for comparison.

refs #117

refs #119

refs #117

Tests now reflect the new RDATA API and use RR type descriptors
  to fill the structures with some data.

Added includes to dname tests.

refs #117

refs #119

Fixed enum dnslib_rr_type - must explicitly set numbers for
  types which do not follow the previous ones in numbering.
Fixed naming of mxrdtln enum items to contain prefix DNSLIB_.

The previous approach made converting RDATA  to wire format too
  complex. Now RDATA items distinguish only between domain name
  and other data. The format is given by a descriptor.

TODO: Consider saving the length of the data to the byte array
      always, so getting it would not require that huge switch.

refs #119

refs #119

Added dnslib_dname_compare().
Added constant for max domain name labels.

TODO: consider saving label offsets in the domain name structure,
      may make the comparation quicker.

refs #118

Was comparing RDATA domain names as domain names, should compare
  as binary data.

Added functions:
  - dnslib_rdata_wire_size()
  - dnslib_rdata_to_wire()

The converting seems to be quite inefficient, should consider
  improving performance for the cost of more memory used
  (not to distinguish so many RDATA item types).

Fixed missing comma.
MAXRDATALEN -> MAX_RDATA_ITEMS
+ added two other constants:
  - MAX_RDATA_ITEM_SIZE
  - MAX_RDATA_WIRE_SIZE

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

Added test for NULL pointers. Todo: possible add more tests with faulty data, questions is, whether such situations can even happen. Refs: #115

Initial implementation of RR type descriptors. It's more or less a simplified copy from NSD, will need changes and maybe more API functions.

Only test for creating RRSet is implemented. Did not try to run it
  nor to compile it. Just an outline yet.

refs #109

Were not checking for NULL pointers.

refs #88

Just basic functions declared:
- creating node
- destroying node
- adding RRSet
- getting RRSet
- getting parent of a node

refs #88

API completed, compilable. Will have to change manipulation with rdata as does not use its API as it should.

RDATAs will be stored in a ordered cyclic list.
Information about RRSet's signatures reflects this.

Added some basic API functions for manipulating the structure and
  comments to describe their intended behaviour.

TODO: Need to define some function for comparing RDATAs.

refs #88

Destructor functions now take ** to set the given pointer to NULL.
RDATA structure now contains pointer to next RDATA, so they may
  create a linked list. (Will be used in RRSet).

Added file comments and \addtogroup statements.

Modified tests accordingly.

refs #88