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.
Merge pull request #43 from stof/refactor_content_type_detection

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