Add Model::has() to see if the criteria exists in the database.

See https://blog.serverdensity.com/checking-if-a-document-exists-mongodb-slow-findone-vs-find/
Merge pull request #45 from jrschumacher/feat-has

Add Model::has() to see if the criteria exists in the database.
Add coverage status - coveralls.io
Add composer badges - https://poser.pugx.org/
feat(Model): extend toArray

- extend toArray to work with referensed models
feat(toArrayRecursive): Add toArrayRecursive
Rename collectionTest.php to CollectionTest.php
Rename inheritanceTest.php to InheritanceTest.php
Rename modelTest.php to ModelTest.php
Update Hydrator.php

Remove unused method hydrateRefs()
Update Model.php

Refactoring the repeated code
Add method processCriteriaWithType() 
Update some annotation
Ignore undefined attribute of model (object instance) when saving
Update README.md

Add content about omitting undefined attribute
Update Model.php
Update README.md
Update .travis.yml
Update database.php
Update Model.php

Fix bug : dirty data of references synchronization
Update Model.php

Fix bug : dirty data of references synchronization
Throw an exception if a reference doesn't define a model