Emails and passwords are not being validated on the server side

[jedwards1211]

This is related to #168.
I thought they were being validated by GraphQLEmailType and GraphQLPasswordType's parseLiteral, but this is actually not the case because the email and password are provided as parameterized query variables, which only go through parseValue.

For instance if I sign up with email "a" and password "a" with curl:
> curl -H "Content-Type: application/json" -X POST -d '{"query": "mutation($email: Email!, $password: Password!) { payload: createUser(email: $email, password: $password) { authToken } }", "variables": {"email": "a", "password": "a"} }' http://localhost:3000/graphql

It succeeds:
{"data":{"payload":{"authToken":"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6IjcwYmMyNTUxLTgzZDYtNDYxOC04ZWMzLTEyYjYzYTFhMTQ0MSIsImlhdCI6MTQ2OTAzMTEyOSwiZXhwIjoxNDY5NjM1OTI5fQ.cS5Zla5DIo4z_HOKGUVHnu0lrqVk4yoEDFsFRWYXc-0"}}}