Add environment overrides.

As you mentioned in pull request https://github.com/ecdeveloper/node-web-crawler/pull/7, having environment overrides is useful. I'm not 100% sure this is the best implementation, but it looks like it will work for us. Any thoughts, comments, objections appreciated!

P.S. The mergeRecursive function I threw in there, which is messy. Perhaps underscore's _extend would be better (http://underscorejs.org/#extend).