Add cmake uninstall target

Change-Id: Id841f695eb0f464cd29ff251b5c03499ba90bce0
Reviewed-on: http://review.couchbase.org/41449
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>