- mostly doxygen comments
- some alignment

closes #159

Refs #159

Refs #159

as null pointers, contained actual key (type of RRset, that is), which did not
comply with definitions, and therefore was producing warnings. Changed node test,
so that they reflect this fact. Skip list's function return_list() and list_length()
now obsolete. Refs #121

fixes #140

Was not initializing the position pointers.

Changes to both rrset and node APIs as well as test for those APIs. Node API and tests should be complete. Refs #108, #109, #110, #116

Checking for bad input parameters in dnslib_rdata_set_items().
Comments adjusted.

refs #119

- wire_size() was not counting the stored length octet in case of
  TEXT or BINARY.
- typo in to_wire()

refs #119

refs #119

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

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

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