Lionfish: methods update

The methods deleteNode and deleteRelationship have
been enriched with return value.
Warning: I've already noticed that deleteNode is
somehow unstable.
Lionfish: popRelationship functionality

There has been added popRelationship to
Lionfish. That method enables developer
to delete one relationship of a given type
without specifying the end node. The method
returns the end node corresponding to the
already deleted relationship.
Merge branch 'origin/lionfish' into spidercrab
Cypher -> Lionfish refactor part III