Comparison of objects: Just compare the keys
improve session setup a bit
notify clients of SQL object creation+deletion

the latter was wrong, the former didn't
Fix object deletion
Add hook for object creation
Extend session creation

users may want their own session handling to not conflict with ours,
so make ours extensible+callable
Add _obsoleted callback for objects to get notified that they're dead
Allow a placeholder for the last objtype key element
Add the ability to specify client methods on sqlobject 'classes'
Rename obj.call_cache to ._call_cache

for hopefully obvious reasons :-)
Improve key handling

* when building a BaseRef object, check the key for scalarity
  (helps find errors)
* make the BaseRef object behave like its key
  item access, sort+hashability
improve field attributes

* a field should behave like its name (hashability/comparison)
* create by name, be type safe
Loaders' .new renamed to .add

since it doesn't actually create anything

Also allow passing in a BaseRef instead of a *tuple
Add a couple of convenience methods for exporting classes

or single objects, for that matter, which just generates a meta for the whole class