Registering a user via POST method

#### What's this PR do?
1. Creates validation rules for the User model
2. Adds additional columns to the schema builder in the User migration file
3. Creates functionality for registering a user when the HTTP POST method is called on the 'users' route
4. Removes destroy method from UserController
5. Adds PHPUnit dependency to composer.php file

#### Where should the reviewer start?
The reviewer should begin in the UserController.php file and review the store() method. The validation rules are passed to the Validator instance which are defined in the User Model. This can be found in the "app/models" directory.

#### How should this be manually tested?
Create a POST request through Curl:
curl --data "email=test@email.com&mobile=7777777777&password=secret" http://homestead.app/1/users

#### Questions
How is the drupal_uid field going to be assigned or generated? Currently a unique value is being assigned using PHP's uniqueid() function.
Perhaps consider making this `doc_uid` field `unique` too.