Refactor User validation.

#### What's this PR do?
- Refactor User validation as described in #12 

#### Testing
Integration with Drupal:
![image](https://cloud.githubusercontent.com/assets/672669/5441603/69dd7d2c-849a-11e4-93fc-fe0fc13efac8.png)

Response example (format stayed the same):
![image](https://cloud.githubusercontent.com/assets/672669/5441632/c2aa1866-849a-11e4-9b25-9fafb3a3b5a8.png)