Provided ability to use JDK ProxySelector

Fixes #360.

* Adds a useProxySelector option that tells async-http-client to use the
  JDK default ProxySelector.
* Adds a ProxyServerSelector interface that AsyncHttpClientConfig and
  ProxyUtils now use.
Merge pull request #362 from typesafehub/proxy-selector

Added support for java.net.ProxySelector