Add HTTP Proxy setting

This pull requests will allow users to specify an http proxy via, in order of precedence, a:
  1) a command line argument or
  2) an environment variable (http_proxy)

Used this SO answer to properly setup http proxy with http nodejs library:

http://stackoverflow.com/questions/3862813/how-can-i-use-an-http-proxy-with-node-js-http-client/6781592#6781592