[MCC-114510] Added AttributeConstraint.

@mdsol/hypermedia-team Added AttributeConstraint. Please review this.
@kenyamat It looks like you've gone back and just added to existing tests. Now this test, which originally only tested the JsonType property, is now testing too much, included the entire Constraint feature. The problem with this is that if any of the constraint assignment operations fails, this test will fail but we won't be able to tell which feature doesn't work. There should be a test for each constraint property. 

Please create a test for each Constraint property.