Add beta 6 info to readme
Removing submodules
Re-adding submodules
Update submodules
Add the tests target
Add operators spec and fixes
Updating submodules
Merge branch 'develop' into feature/abstractify

* develop:
  Updating submodules
Add SNPConstraintProducer protocol
Add SNPAttributeType protocol
Change SNPConstraintProducer description
Add ConstantType to ConstraintProducer
Remove SNPAttribute base class
Update SNPExpression class
Merge ConstraintProducer and AttributeType
Update xcodeproj
Further generalize SNPExpression
Operators overhaul
Remove usage info from SNPOperators
Remove unnecessary SNPVersion file