Don't export dom module if on node

[sohkai]

Would be nice to only export the dom module if the user's targeting the browser, similar to how fetch-ponyfill and others do it.

Currently, including the entire package through require('js-utility-belt') on node results in errors because node doesn't have window or document.

From the looks of it, this should be easily done by using the package.json's browser field for lib/index.js and changing the main field for a lib/node.js that strips away the dom export.

[sohkai]

See webpack/webpack#1122 for some ideas for webpack building.