Source files reorganization.

- Sources separated according to the future compilation modules:
  - src/knot/ contains all sources related to server (compiles into
    knotc and knotd)
  - src/alloc/ contains sources of slab allocator (may be compiled
    into standalone library).
  - src/dnslib/ contains dnslib sources (may be compiled as
    standalone library).
  - src/tests/ contains unittests (compiles into unittests).
  - src/zoneparser/ contains sources of zone compiler (compiles
    into knot-zcompile).

TODO: Need to refactor makefile to separate the compilation of
      these modules. However, unittests still depend on all
      modules.

refs #400