Change device registration to PUT

Looking here:
https://github.com/aerogear/aerogear-unifiedpush-server/pull/543#issuecomment-99555746

it seems reasonable to do a change of the API.

It can be done on 1.2.0 - or even later at a 2.0.0 phase of the server.

Note: this requires a lot of updates:

• server
• all client SDKs
• docs

And updates to (integration) tests