Using default handler for before and after request signals.

In the before_request default handler, the Response object is
initialized. This way, the Response object is still uninitialized for
connected handlers.

In the after_request default handler, the Response body is completed.
This way, anything registered after_request may still edit the body
before it gets completed.

For both, handlers can be connected after using connect_after.
Merge pull request #13 from arteymix/complete-response-body-in-default-handler

Using default handler for before and after request signals.