Setup for testing

[johanbrook]

Branched off #78.

This patch:

• Adds package tests for Mailer. Just to check for existence of Mailer object for now.
• Sets up example app for testing in app testing mode. Checks if an email renders with data.
• Use ES6 modules.
• Runs tests on CircleCI.
• Updates example (and now also test app) to Meteor 1.4.2.

[johanbrook]

.. and of course Meteor's isobuild breaks somehow when building on CI, probably because of meteor/meteor#7711.

Edit: Yay, fixed in 905e1b0.