Reduce size from 12K minified+gzip to 1K minified+gzip when included via Browserify

I'm including node-uuid in http://derbyjs.com/ using Browserify.

Browserify will shim standard Node.js packages including `require('crypto')` and globals including `Buffer`. Since node-uuid already works without these features, including them dramatically bloats the size of the bundle produced by Browserify by about 12X. In addition, I didn't test the performance when included this way, but it is likely worse than the Browser native implementations already in node-uuid.

The pull request simply prevents Browserify from recognizing these items by accessing them from `_global`.