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 browserify `this` refers to the `exports` object rather than the window. They do this to be compatible with Node.