Add environment support

This version adds support for specifying the production or sandbox environment in the environment.js file of your Ember app to support the Sandbox beta.

module.exports = function(environment) {
  var ENV = {
    modulePrefix: 'host',
    environment: environment,
    baseURL: '/',
    locationType: 'auto',
    EmberENV: {
      // ...
    },
    APP: {
      // ...
    },

    squarePaymentForm: {
      environment: 'sandbox'
    }
  };
};

If you have a custom URL to the Payment Form library, you can use the jsUrl parameter to specify it:

    squarePaymentForm: {
      jsUrl: 'https://example.com/square-js-lib'
    }