Refactored the content-type detection to use the BrowserKit hook point

By using the BrowserKit extension point rather than the Goutte extension point, we don't use the Guzzle classes directly anymore in our extended client.
This will make it much easier to support both Goutte 1.x and 2.x at the same time. Actually, this change already introduce the support of Goutte 2.0 in the driver. The only reason why I haven't changed the constraint is because I want to update the MinkExtension first to support configuring Goutte 1.x and 2.0 (depending on the installed version), and this requires a bit more time than I have this evening (it may have some impact on the userland configuration).

this replaces https://github.com/Behat/MinkGoutteDriver/pull/34