Remove obsolete code
Revert "Fix the way Form data is added to the request"

This reverts commit 52545a2200b3031a2b817b7e67bd06d45931a003.

Bring back BDataIO as there is no good way to use our BHttpForm class.
WebKit already does all the http encoding and formatting work.
Add more dependencies to the README.
Use modified Haiku Service Kit API

 * Rename BUrlProtocol to BUrlRequest
 * It is now possible to set the size of a request even when data is
read from a BDataIO, make BFormDataIO use this
 * Temporary setup of a static BUrlContext in BUrlProtocolHandler to
store cookies. Our WebKit cookie API will need some changes to work
completely.

This gets POST data working the right way again.
Rework and fix cookie handling

 * The previous implementation of the cookie jar didn't allow to
synchronize the cookies seen by Javascript and set in meta tags with the
ones sent and received in HTTP headers. This made many JavaScript
oriented websites fail.
 * Since the cURL backend is removed, drop the now useless CookieJar
implementation, and instead use the Service Kit one directly. This makes
the integration of WebPage in the official Haiku API make a bit more
sense.