Add `browser` field to `package.json` for webpack/browserify support 

#25 added respimage to `npm`.  Awesome, but sadly `require("respimage")` will fail unless `package.json` contains a `main` or `browser` field.

By convention, `main` is the Node-compatible build and `browser` is the browser-compatible one.  Since respimage is useless in Node, I've populated `browser`.