This increase the safety of our build; external consumers
do no longer need to account for the `browser` field to
bundle bpmn-js (or otherwise bundle a Node shim, unintentionally).
* Treat features as optional dependencies and register action
only if feature exists
* Explicitly add features to the Modeler and NavigatedViewer types
* use ES6 import / export
* UTILS: export individual utilities
* TESTS: localize TestHelper includes
BREAKING CHANGE:
* all utilities export independent functions
* library sources got ported to ES6. You must now use
a ES module bundler such as Browserify + babelify or
Webpack to consume this library (or parts of it).
