add config parameter to use a local web proxy (with health checking)

Adds support for the following new config parameters:

:use_local_web_proxy (defaults to false)
:local_web_proxy_port (defaults to 8123, polipo default)

If :use_local_web_proxy is enabled, the code will attempt to connect to that proxy and verify that it's working. The external address IP address of the host computer is determined by inspecting the default gateway. If it is working it will be used, if it's not working it will be disabled.

http_proxy and https_proxy work well for organizations that run a centralized cache, but don't serve use cases where the developer runs a local cache well.

Maximum speed gains are gained by rewriting the urls in the install.sh from https to http to enable caching (proxies that work with SSL content only allow it to pass -- they cannot cache it). See the master branch for that modification. Ideally the install.sh would just be changed to http links (the advantages of using HTTPS aren't clear).

///

.kitchen.yml snippet

driver_plugin: vagrant
driver_config:
  use_local_web_proxy: true
  # local_web_proxy_port: 3128 # squid default port