Add http\Client::setDebug(callable $debug) and constants

Returns:
 * http\Client, self

Params:
 * callable $debug as
   function(http\Client $c, http\Client\Request $r, int $type, string $data)

Added constants (used for the $type bitmask parameter of the callback):
 * http\Client::DEBUG_INFO (informational text)
 * http\Client::DEBUG_IN (received data)
 * http\Client::DEBUG_OUT (sent data)
 * http\Client::DEBUG_HEADER (headers)
 * http\Client::DEBUG_BODY (body)
 * http\Client::DEBUG_SSL (SSL data)

E.g. incoming headers would have the bitmask
(http\Client::DEBUG_IN|http\Client::DEBUG_HEADER) set in the $type
parameter.

Messages of type http\Client::DEBUG_INFO contain human readable text
like "Proxy CONNECT connection closed\n".