Fixed dname API, added 3 functions, fixed impl.

dname and node were referencing each other. Used
  forward-declaration of node in dname.
Added API getter functions (with implementation):
  - dnslib_dname_name()
  - dnslib_dname_size()
  - dnslib_dname_node()
Fixed dname implementation:
  - str_to_wire() was replacing chars by label lengths
  - new_from_str() bad assert
  - new_from_wire() was not allocating space for the name
  - to_str() was de facto copying the wire format

Added some debug output to dname

refs #5, #88