Added client side communication to the Subsurface Web Service

A couple of new files webservice.c and webservice.h are added.

webservice.h exposes two methods at the moment:
- webservice_download_dialog():
this function creates the user interface for the download dialog
from the web service.
- webservice_request_user_xml()
this function is a direct call to retrieve XML for a specific
user identifier. the actual data, data length and error codes
are stored in passed pointers.

A menu entry is added in the Log menu:
"Download From Web Service"

The used backend for communication at the moment is provided
by libsoup.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Updated the Makefile to support libsoup and include webservice.[c|h]

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>