Fix browserify compatibility

Incompatibility was mostly related to the way browserify determines and implements required modules:
 - it doesn't support dynamic require substack/node-browserify#377
 - considering setImmediate not to be a part of ECMA substack/node-browserify#985
 - lacks some exports expected from `http` (along with `https`) module https://github.com/substack/http-browserify/pull/75

Fixes #150
Depends on https://github.com/substack/http-browserify/pull/75