Implement asprintf & vasprintf for win32

Change-Id: Iaa40c48b021e5ef493b49741e06fb3c22c0118e0
Reviewed-on: http://review.couchbase.org/40828
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>