Merge pull request #663 from hintjens/master

Fixed random failures in test_security_plain and _curve
Merge pull request #664 from hintjens/master

NULL mechanism was not dealing with ZAP denials properly
Merge pull request #665 from hintjens/master

Added test case for NULL domains
makes curve keys symetric as in libcurve + factorisation
oblige the application to explicitely set the node type for PLAIN
Merge pull request #666 from lalebarde/master

curve key names as in libcurve, symetry design, a few logic improvements
Reformatted text
Revert "oblige the application to explicitely set the node type for PLAIN"

This reverts commit 1206f4572b4b3bb405a14cc60605b75d53b819a0.
Revert "makes curve keys symetric as in libcurve + factorisation"

This reverts commit bfd472f97cad5e37fa384b687a906f515600bac4.