Add '.js' to 'browser' field file names.

From some trial and error, it seems like files in the ['browser' package.json field](https://github.com/substack/node-browserify#browser-field) need to include the file extension to work properly. Before changing this, I always got the output from `./src/node.js`; now it seems like it working.

Also, I removed the `var frames = global.frames` line from `./src/browser.js`, as there is no reference to a `frames` variable.