Enable proxy option?

[avrombay]

This might be too specialized, but I think adding a feature so people can specify a proxy server ala CURLOPT_PROXY would be a great idea, especially in situations where it would be advantageous to cache certain requests in a webcache like squid or varnish.

I'm going to write a EchoNest_HttpClient_ProxyCurl class to test it out, unless you have any objections.

Thanks!

[bshaffer]

That sounds like a great idea! Please submit a pull request when you are finished.