Add webpack support

[cristiano-belloni]

This adds Webpack support.
There are two faces of this:

1 - Add the browser entry to package.json, as described in #22
2 - jailed.js fails with latest Webpacks, because the process variable is defined. I added another test to check if process.browser is false, before we decide we are on Node.

[asvd]

okay, thanks!