Implement provider.json caching
Right now we get the provider.json file always from https://provider.domain/provider.json.
We pinned that connection to use the EE certificate of the provider, but this causes problems when the cert expires or is revoked.
We're going to insert a provider.json file in the app, letting it use the api to update the file, so that we can use the certificate pin defined inside provider.json.
This implies that the providers must keep old APIs usable until every client has been updated (with the new provider.json inside the app). To ease this task, we'll regularly download the file via API so that people using the app get the new version as soon as possible.
(from redmine: created on 2015-06-09)