Added CREATE/DROP CONSTRAINT syntax in Cypher

o Create contraint syntax, for example:
  CREATE CONSTRAINT ON (foo:Foo) ASSERT foo.name IS UNIQUE
o Drop constraint syntax, for example:
  DROP CONSTRAINT ON (foo:Foo) ASSERT foo.name IS UNIQUE
o Doesn't actually do anything, just able to parse it.
Merge pull request #747 from tinwelint/2.0-cypher-constraint-syntax

Added CREATE/DROP CONSTRAINT syntax in Cypher