Default to doing DNS resolution through a proxy server if present.

- Resolve an issue where proxy information by default was not resolving DNS
  through a proxy server (unlike the `socks` library), so users getting
  proxy information from an environment variable (HTTP_PROXY/HTTPS_PROXY)
  with no external DNS could not connect to hosts with DNS names.

- Resolve a potential issue where if the proxy server was a different
  address family to the destination host and there was functional DNS, may
  not work.  For example, using a IPv4-only proxy server to connect to an
  IPv6-only host via an IPv4-only internal network.

- Improved documentation of the ProxyInfo class.
Merge pull request #276 from micolous/master

Default to doing DNS resolution through a proxy server if present.