Use ES6 modules

[danvk]

This PR migrates dygraphs from its own ad-hoc dependency system to the new ES6 module system.

Highlights:

• We use babel to transpile ES6 code to the ES5 code that browsers like
• We use browserify to build bundled JS for the browser. Since browserify isolates each module for us, there's no more need for IIFE wrappers.
• Each file is now its own distinct module, with its dependencies listed explicitly. No more defining properties on Dygraph from a dozen different files.
• We use envify to set the DEBUG property to distinguish dev/prod builds.
• Tests can be run & iterated on using the mocha HTML test runner and a new watch.sh script.
• DEFAULT_ATTRS gets moved into its own module.
• Deleted dygraph-combined.js from source control (fixes Remove dygraphs-combined.js from source control #376)
• A few functions (e.g. numberAxisLabelFormatter) move out of dygraph.js and into dygraph-utils.js. We need to shrink dygraph.js, it's huge.

Fixes #453

[timeu]

Grest to see that dygraphs is moving forward.
I am wondering if it still will be possible to generate a dygraphs-combined.js to include for example for a GWT wrapper?

[danvk]

Running scripts/build.sh from master should produce dist/dygraph.min.js and dist/dygraph-combined.js (which are the same thing—I'd like to move to the .min.js naming convention for 2.0). It'll also make dygraph-gwt.jar.