Validators for User Model

I fixed a small bug in user_controller.rb. the variable @user was not being set in the create function.

Added these validators to the user model
- Only alphanumeric characters
- Minimum 5-character password
- Maximum 32-character password
- Minimum 3-character username
- Maximum 20-character username
- Validate the format of email

Removed
- Checking pressence of password and username. (Length checking took care of it)