browserify compartibility

This PR makes picturefill compatible with browserify (and webpack). There is a similar open pull request (#252) but this one makes the tests pass by adding `"browser": true` to `.jshintrc`. It also adds a `main` field to `package.json` to allow installing it via npm.
In which environment is picturefill supposed to run that doesn't have a global `window` and `document` variable? Saying that, we could also change `window.document` into just `document` which is what most libraries do that wrap their code into an IIFE.