Abstract prefs storage code into Horde_Prefs_Storage.

Abstract prefs storage code into Horde_Prefs_Storage.

Jan and I talked about using a stack system for retrieving prefs data
from storage.  After discussion, determined that cache storage won't
work in the stack system for several reasons.  However, it may be
possible to implement otherwise.  For example, in Horde, there would be
2 stack drivers.  The first is a read-only driver that returns the
default values from the prefs.php files.  The second would be the
persistent storage driver.