Provide sane defaults for gateway URL and postage stamp id

[agazso]

Most developer who try out the library will want to test their code as soon as possible without having to bother with installing components and setting them up.

The idea is that we could provide sane defaults for the api URL with https://api.gateway.ethswarm.org and the default postage stamp id 0000000000000000000000000000000000000000000000000000000000000000 as exported constants.