Firebase delegation example.
[skip ci]
Constant for Firebase URL.

[skip ci]
Merge pull request #50 from auth0/chore-firebase-example

Firebase delegation example.
Remove unnecesary constraints.
Username validator.
Email validator.
Password validation (non empty).
Confirm password validation.
Credentials validator.
More tests.
Use new validators in Sign Up.
Use new validators in auth screens.

Remove old validators.