Enable use of a proxy server

I have made changes to allow the user to specify details of a proxy  server using two new config parameters: aws.proxyServer and aws.proxyPort.

If these are present then the proxy server is set on the WsRequestHolder when it is created by a call to AwsRequestBuilder.url. If no proxy is specified then the behaviour is the same as before the change.

I have added a new test, AwsProxySpec, to test that the proxy is correctly set. All other tests still pass as before.