Refactor raven-js to use CommonJS modules, class instances (2.x)

[benvinegar]

This converts raven-js to use CommonJS modules, internally. Distributables are built with Browserify.

(Same as #421, but made against the 2.x branch.)

Public API changes

• For anyone using the CDN versions of Raven, nothing changes. Plugins automatically install as they did before.
• If you are using Raven w/ CommonJS (e.g. Webpack or Browserify), plugins are now loaded like so:

var Raven = require('raven-js');
Raven
  .config('YOUR DSN')
  .addPlugin(require('raven-js/plugins/angular'))
  .addPlugin(require('raven-js/plugins/jquery'))
  .install();

Internal changes

A lot of internal changes that shouldn't affect users. Most notably, Raven itself is now a class instance, and most previously-private variables and functions are accessible as "private" object properties (e.g. Raven._globalOptions and Raven._handleStackInfo). They're aliased with an underscore for a reason – you shouldn't access these or assume anything about them.

This also means that Raven's API methods must be called with Raven as the this value. This could impact users if they have been aliasing Raven API functions directly, and not invoked them on the Raven object itself.

e.g.

// THIS WILL NO LONGER WORK
var Raven = require('raven-js');
var config = Raven.config;
var install = Raven.install;
config('YOUR DSN'); // "this" is window, not Raven
install();

Hopefully nobody does this.