Test freeGlobal.self for browserified web workers.

`self` is the global context name in web workers. So
`freeGlobal.self === freeGlobal` is `true` in that environment when
lodash is packaged in a library/site for use in web workers.
Merge pull request #458 from mzgoddard/support-browserify-webworker

Add `freeGlobal.self` check to to support browserified code running in a web worker.