DOC: Updates to Introduction/Installation.

Change-Id: I294b4b7eacad11b6ce35ea155cffdaaf97a8ff71
BUG: Turn BUILD_TESTING ON in the superbuild.

It is needed to download required data.

The ITKReview module also needs to be ON.

Change-Id: Ie18f8be66b7309c7816e875cbd128434709f60f3