First usable version of a canvas-less paperjs

I've added a `worker` option to the environment options that allow to generate a `paper-worker.js` dist file.
This version of paperjs can be initialized without a canvas element and be used inside a WebWorker:
```js
paper.setup({
	width: 1024,
	height: 1024
});
```
There are numerous methods of the API as well as some objects that become irrelevant/useless without a canvas (such as `.update()` or `.rasterize()`) and using them will probably throw.

The work isn't over, there might be some bugs with parts of the API that I haven't used yet. But I wanted to get your feedback on these environment options. FYI, there's a demo of plumin.js that uses paper-worker.js and is able to build typefaces in a WebWorker: http://byte-foundry.github.io/plumin.js