Add Symfony Validator Service

## Add Symfony Validator Service

### Acceptance Criteria
1. Service is added via `ValidatorServiceProvider`.
1. `"symfony/validator": "~2.3"` is required in `composer.json`.
1. `AbstractEntity` has `public function getValidationRules()` to return entity validation rules. (Returns an associative array of `Symfony\Component\Validator\Constraints` objects. [See documentation](http://silex.sensiolabs.org/doc/providers/validator.html#validating-associative-arrays).)