Switch to opening storages by URL

Previously for specifying a storage we were requiring users to create
zconfig file and put information about storage there. This is not always
convenient e.g. for quickly inspecting a file storage or a NEO instance
here and there for which address and name is already at hand.

So thanks to zodburi we can switch to specifying storages by URL without
loosing generality as there is still zconfig:// schema which allows to
configure storages zconfig way.

P.S. for convenience we allow paths without schema to be treated as
    FileStorage (i.e. file:// schema is implicitly used).

P.P.S. zodbanalyze is not affected (for now ?) as it currently
    works with FileStorage only.

[1] http://docs.pylonsproject.org/projects/zodburi/