implemented zonemd calculation/verification

Knot.files

Knot.includes

doc/doxygen/Doxy.page.h

src/knot/Makefile.inc

src/knot/zone/digest.c

src/knot/zone/digest.h

src/libdnssec/Makefile.inc

src/libdnssec/digest.c

src/libdnssec/digest.h

src/libdnssec/error.c

src/libdnssec/error.h

src/libknot/Makefile.inc

src/libknot/libknot.h.in

src/libknot/rrtype/zonemd.h

tests/.gitignore

tests/Makefile.am

tests/knot/test_digest.c