Improve the generated visual studio projects

Adding the headerfiles makes them visible in the solution
browser

Change-Id: I4b73a2a56e01df41bf6dcdb38815345cdccac6ef
Reviewed-on: http://review.couchbase.org/33931
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>