Axiom

- Updated axiomatic to work with Twisted trunk.

- Raise a more informative exception when accessing Item references pointing
to nonexistent items.
- Enforce prevention of deletion of items referred to by references set to
reference.DISALLOW.
- Tables in the FROM clause of SQL generated by queries are now ordered by the
order of the Item subclasses in the comparisons used to generate them.
- A new IComparison implementation has been added to allow application-level
code to explicitly specify the order of types in the join.

- Added a 'batchInsert' method to Store, allowing insertion of items without
loading them into memory.
- Change ItemQuery.deleteFromStore to delete items without loading them if
possible.

- Changed userbase.getLoginMethods to return LoginMethods rather than
(localpart, domain) tuples.

- Fixed a scheduler bug that would cause tasks scheduled in a substore to be
removed from the scheduler.

- dependency.dependsOn now takes similar arguments to attributes.reference.