Support ES6 or CJS style of runtime

In the client, the developer may be using either the CommonJS style of Handlebars (with exports = Handlebars),
or the ES6 module style (with exports.default = Handlebars).

In order to support either style, attempt to use ES6 style with "default" exports, otherwise fall back to CommonJS style.
Adds tests for different runtime exports styles
Merge pull request #12 from diurnalist/feature/es6-or-cjs

Support ES6 or CJS style of runtime