Brand information API

[daniele-athome]

Provide an in-band extension to retrieve provider information to be displayed in the app splash screen.

Following #77, this needs to be done on two levels (since sponsors may differ):

• server brand (server list + another ad-hoc command for on demand information)
• registration provider brand (registration response)

The first idea was to make it only in the server list command, but on second thought it could be useful to also add this information to the registration protocol (i.e. some more data form fields on the reponse) for the registration provider brand, and via another ad-hoc command for the server brand.

Ref. kontalk/androidclient#518.