browser support

this isn't done yet, but i'm opening this to keep track of progress

todo for browser support:

- [ ] use https://npmjs.org/package/bops everywhere Buffer/Typed Arrays are used instead
- [ ] move examples/slicer/test.js into it's own module (probably called `psd-preview`)
- [ ] write a simple little integration test that can be easily run in both node + browsers (e.g. via `npm test` or `npm start` to start a browser test server)