Add an initial set of dependency injection bindings.

Add an initial set of dependency injection bindings.

These are configured automatically when the Registry is set up, access the
current global $conf array, and let you create:
- a Horde_Db instance, by asking for db-reader, db-writer, or db-manager (the
  distinction isn't implemented yet)
- a null Horde_Log_Logger object (our config is still for PEAR Log)
- a real Horde_Cache object as configured in $conf