[FEATURE] Add a download method to t3lib_http_Request

To ease work for developers, add a download method to
t3lib_http_Request. This disables the body, attaches a download
observer and sends the request.

Change-Id: I4123efbd48cb50b82510f9b9e08cfbd92d6090bc
Resolves: #28495
Releases: 4.7
Reviewed-on: http://review.typo3.org/3729
Reviewed-by: Stefan Neufeind
Reviewed-by: Oliver Klee
Tested-by: Wouter Wolters
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch