Added support for Constraints to Cypher

Hi, I needed the functionality to create/drop constraints so I've added basic support for it to Cypher.

There are now two fluent methods: CreateConstraint and DropConstraint

CreateConstraint("book:Book", "book.isbn") is equivalent to:
CREATE CONSTRAINT ON (book:Book) ASSERT book.isbn IS UNIQUE

DropConstraint("book:Book", "book.isbn") is equivalent to:
DROP CONSTRAINT ON (book:Book) ASSERT book.isbn IS UNIQUE

http://docs.neo4j.org/chunked/milestone/query-constraints.html

Cheers!