redesigned schema to allow more flexible use

What should now work:

 *  Something like RBAC on per-endpoint basis.
 *  Composite primary keys (at least on virtual tables).
 *  Endpoint names different than names of database tables.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
implemented POST method for collections

Signed-off-by: Jan Dvořák <mordae@anilinux.org>