Add support for no_proxy environment variable when using http_proxy

Faraday currently supports the `http_proxy` environment variable and sets it as a default proxy if another is not explicitly set when a connection is created.

The `no_proxy` environment variable contains a comma-separated list of domains for which the proxy is not required[1]. If the requested URL matches an entry in this list, the default proxy should not be used.

This patch adds support the `no_proxy` behaviour.

[1] This is already supported in a large number of existing ruby projects, e.g. [RubyGems](http://docs.rubygems.org/read/chapter/13#page51)