Added modules Zone database & Zone data structure.

Zone database - Now dummy module, no functions implemented yet.
  Later will group all zone data structures, and provide high-
  level API for the other server modules.

Zone data structure - Only wrapper around hash table functions.
  Should provide transparent API for Zone database, disregarding
  the underlying data structure.

Zone node - data structure to hold all information associated with
  a domain name.
  Consider renaming this structure later.
  Used in Zone data structure and through its zds_node structure
  in the rest of the server modules.
  The use of zds_node structure needs revision, maybe useless.

New (now dummy) types for domain name (dnss_dname) and wire format
  of the domain name (dnss_dname_wire). Both typedef'ed to char *.
Convenience methods to get string from dname and to get length of
  a dname. Will be changed later, when the types change.

Minor changes (comment style changed).