v4.0.0

#304 Return points, spelling and dead code
#308 Add support for type coercion
#310 performance improvements
#314 Updated to check that $ref is a string
#316 fix(NumberConstraint): force positive value on first arg
#317 Add support for $ref on properties
#318 chore(phpunit): ignore local phpunit.xml
#319 feat(demo): add simple demo script