Added new API to download images remotely

Added new API to download images remotely

Added "DownloadImageSynchronously()" method, which invokes libcurl to
download a URL.

Added global object to ensure curl is globally initialized and cleaned up,
this should allow usage of the above method in multiple threads simultaneously.

Also added "GetClosestImageSize" method, which partially loads the image file
to determine it's size.

Change-Id: Ie27f69d729abecfe5de3d5a90eae01a34f92c5b1
Signed-off-by: David Steele <david.steele@samsung.com>