Add ability to set any response header and explicit support for cookies.

The 4th line in the response mocktail file used to specify the content type.
Instead now all lines from the 4th line until an empty new line will be treated
as response headers and parsed accordingly (i.e. will expect Name: Value format).
All data after the empty line will be treated as response body as its been before.

I needed this change to test my iOS app which needs a login cookie and thought
others might find it useful too.