New Features

Model.dataset_module has been added for easily adding methods to a model‘s dataset:
```
  Album.dataset_module do
    def with_name_like(x)
      filter(:name.like(x))
    end
    def selling_at_least(x)
      filter{copies_sold > x}
    end
  end
  Album.with_name_like('Foo%').selling_at_least(100000).all
```
Previously, you could use def_dataset_method to accomplish the same thing. dataset_module is generally cleaner, plus you are using actual methods instead of blocks, so calling the methods is faster on some ruby implementations.
Sequel now uses a Sequel::SQLTime class (a subclass of Time) when dealing with values for SQL time columns (which don‘t have a date component). These values are handled correctly when used in filters or insert/update statements (using only the time component), so Sequel can now successfully round trip values for time columns. Not all adapters support returning time column values as SQLTime instances, but the most common ones do.
You can now drop foreign key, primary key, and unique constraints on MySQL by passing the :type=>(:foreign_key|:primary_key|:unique) option to Database#drop_constraint.
The ODBC adapter now has initial support for the DB2 database, use the :db_type=>’db2’ option to load the support.

Other Improvements

The mysql2 adapter now uses native prepared statements.
The tinytds adapter now uses uses sp_executesql for prepared statements.
DateTime and Time objects are now converted to Date objects when they are assigned to a date column in a Model instance.
When converting a Date object to a DateTime object, the resulting DateTime object now has no fractional day components. Previously, depending on your timezone settings, it could have had fractional day components.
The mysql2 adapter now supports stored procedures, as long as they don‘t return results.
Mass assignment protection now handles including modules in model classes and extending model instances with modules. Previously, if you defined a setter method in a module, access to it may have been restricted.
The prepared_statements_safe plugin now works on classes without datasets, so you can now do the following to load it for all models:
```
  Sequel::Model.plugin :prepared_statements_safe
```
Dataset#hash now works correctly when handling SQL::Expression instances.
Model#hash now correctly handles classes with no primary key or with a composite primary key.
Model#exists? now always returns false for new model objects.