=== New code organization

Sequel is now divided into two parts: sequel_core and sequel_model.
These two parts are distributed as two separate gems. The sequel gem
bundles sequel_core and sequel_model together. If you don't use
Sequel::Model in your code, you can just install and use sequel_core.

=== New model hooks implementation

The hooks implementation have been rewritten from scratch, is much
more robust and offers a few new features:

* More ways to define hooks: hooks can now be defined by supplying a
block or a method name, or by overriding the hook instance method.

* Inheritable hooks: Hooks can now be inherited, which means that you
can define general hooks in a model superclass, and use them in
subclasses. You can also define global hooks on Sequel::Model that
will be invoked for all model classes.

* Hook chains can be broken by returning false from within the hook.

* New after_initialize hook, invoked after instance initialization.

* The hook invocation order can no longer be changed. Hooks are
invoked in order of definition, from the top of the class hierarchy
(that is, from Sequel::Model) down to the specific class.

=== Miscellanea

* Removed deprecated adapter stubs, and all other deprecations in both
sequel_core and sequel_model.

* Fixed String#to_time to raise error correctly for invalid time
stamps.

* Fixed error behavior when parse_tree or ruby2ruby are not available.

