add support for proxy servers

This resolves issue #12 and adds support for HTTP proxy servers in the Keen IO client. 

A couple of notes:
    1) It does not support authenticated proxies. users can set the authenticator on their own
    2) It currently only supports HTTP proxies (this is the vast majority of users)
    3) Not setting the proxy does not block configuring a container level one.

A note on code:
    I picked constructor injection to get the proxy into KeenHttpRequestRunnable. This was a personal preference. If you would rather I directly reference the static config at the point where it is used I can change it.