Use Guzzle components rather than the full-stack

This changes Goutte so that it only requires the Guzzle HTTP component rather than the full-stack, dramatically reducing the size of the PHAR (~3.5MB to under 1).

The history/mock Guzzle plugins are dev-required.

The BC "break" here would be if someone was using other parts of Guzzle, but they should be listing them as their own dependencies anyhow.