Modify data structure

* Add serial_number for a composite primary key
  to instances. This is due the fact that uuids
  are not sufficiently random in large scale.

* Fix data types and sizes.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Update README

* Add openssl and httpd-tools dependencies.
* Change clone url.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>