prevent loading the the `fs` module in the browser

while browserify uses an empty module for `fs` by default, webpack does
not and will refuse to bundle fabric (previous versions of webpack did
bundle it, but that was considered a bug in webpack which was recently
fixed (webpack/webpack#411).
Merge pull request #1620 from c089/prevent-loading-fs-module-in-browser

prevent loading the the `fs` module in the browser