Modularize the codebase and use ES6 features with Babel+browserify+gulp build

[taye]

This branch splits the monolithic ./interact.js file into separate modules to be bundled together with browserify. It also makes actions (drag, resize, gesture), modifiers (snap, restrict), pointerEvents (down, tap, hold, etc.) and autoScroll optional and provides interfaces for adding new actions and modifiers.

An internal signals API been introduced which allows modules to hook into various points in the codebase without tight coupling, eg.:

signals.on('interactable-new', function (arg) { ... });

// in Interactable constructor
signals.fire('interactable-new', {
    interactable: this,
    element: element,
    options: options,
    win: _window
});

Eslint is now used for style checking instead of JSHint and the formatting style of the project has been changed. Most notably:

• 2-space indentation instead of 4
• the use of various ES6 features (see .babelrc)
• let and const instead of var
• only one let/const per identifier
• multi-line trailing commas

One-time setup:

• install Node.js if you don't have it
• clone the interact.js repository
• enter the directory into which it was cloned
• on the command line, run npm install to install interact.js' development dependencies

To build interact.js, run:

npm run build

This will generate the following files in the ./dist directory:

interact.js
interact.js.map
interact.min.js
interact.min.js.map

Many thanks to @stephen-james for his help!

Closes #110