Axiom

- Fixed docstrings in axiom.dependency.
- Scheduler and SubScheduler now declared to implement IScheduler.

- Multi-version upgraders are now supported: an upgrader function
can upgrade items more than a single version at a time.
- Multi-item-class queries now supported: Store.query takes a tuple
as its first argument, similar to a comma-separated column clause
for a SELECT statement in SQL.
- Empty textlists are now properly distinguished from a textlist
containing a single empty string.
- Handling of items scheduled to run with axiom.scheduler being
deleted before they run has been fixed.

- AxiomaticCommand is no longer itself an axiom plugin.
- axiom.test.historic.stubloader.StubbedTest now has an
'openLegacyStore' method, for opening the unupgraded store
multiple times.
- The default argument to Store.getItemByID is now respected in the
case where an attempt is made to load an item which was created
and deleted within a single transaction.

- A new method, axiom.store.ItemQuery.paginate, has been added, which splits
a query's result-gathering work into multiple "pages" so that we can deal
with extremely large result sets.
- A dependency management system for Items has been added in
axiom.dependency. InstallableMixin has been removed;
axiom.dependency.installOn is now used to install Items and connect powerups.
Items can declare their dependence on another item by declaring attributes
with axiom.dependency.dependsOn. When items are installed, their dependencies
will be created and installed as well. Installation is no longer tracked by
'installedOn' attributes but by _DependencyConnector items.
- A bug preventing 'axiomatic userbase list' from working on a fresh
mantissa database has been fixed.

- Change ItemQuery.deleteFromStore so that it will call deleteFromStore on an
Item subclass if it has overridden that method.

- Added fullyQualifiedName to IColumn, _StoreIDComparer, and _PlaceholderColumn.
- Added support for distinct Item queries and for counting distinct attribute
queries.
- Exceptions raised by Axiom upgrade methods are logged instead of silently
swallowing them.