Added the constraints boolean, isArray and keyExists
Added the constraint notEmptyKey and completed the issue #4
Added the constraints object, instance and notInstance. Added too the method getValidation to Validator for get a single violation message based on id
Added a new constraint: date
Updating the composer format
Merge pull request #10 from oandre/develop

Updating the composer format
Update README.md
[WIP] Adding the custom constraint registration feature from issue #11
Merge pull request #13 from project-melody/develop

Preparing release 1.1.0