Provides fields validation for the User model.

#### What's this PR do?
- Adds [Laravel Model Validation](/JeffreyWay/Laravel-Model-Validation) composer package
- Introduces user fields validation
- Makes `Email` unique as required per #10.
  Username is generated from email, so it'll be unique as well

#### Testing
This PR consistent with the last DS Canada integration:
![image](https://cloud.githubusercontent.com/assets/672669/5114157/ea9480ac-703e-11e4-9a20-733d54983543.png)

#### Any background context you want to provide?
Composer package added, so `composer update` is required during deployment.
This must be deployed after DoSomething/dosomething#3480 is closed and the relevant code is running on DS production.

#### What are the relevant tickets?
Laravel part for issue #10.