Break a lot of stuff. Also, database redesign.

See here: http://valodim.stratum0.net/okc/db_new.png

Change entire database design. This introduces a lot of changes,
notably all URIs and almost all projections. Some features (like key
deletion) have been simply commented out for now since they need
serious reconsideration. There are a couple of new TODOs marking places
where more work is needed.

The migration path works fine from what I tested. The old database is
not deleted for now, ie, it is reimported at every start of the
application making all intermediate changes transient.

Tested and working (for me):
 - All activities in the main drawer
 - Multiselect and Search
 - ViewKeyActivity, with and without secret key available
 - CertifyKeyActivity
 - SelectSecretKeyActivity (from CertifyKeyActivity)
 - SelectPublicKeyActivity (from encrypt activity)

What doesn't work:
 - Actually certifying keys (pending a TODO in ProviderHelper)
 - Importing keys doesn't preserve secret keys
 - "Encrypt to this contact" doesn't pass key
 - Editing keys. All controls are disabled, I'm not sure why... (is this
   even my fault?)
 - Deleting keys

What I didn't test:
 - Key export
 - API stuff
 - Creating keys (since editing doesn't even work)