If your project uses a date library that doesn't extend \DateTime (e.g. Zend_Date), then you
currently have to convert to and from ActiveRecord\DateTime when interacting with models, which is a
pain and error prone. We've had to fix quite a number of bugs due to missing conversions. This
commit removes the need for that by adding support for using a custom date class.

The custom class is set in ActiveRecord\Config, which uses similar logic to the logger methods for
the getter and setter. The ActiveRecord\Config->set_date_class() method checks if the provided class
has "format" and "createFromFormat" methods, which are all that are required.

I changed everything that created new date objects to use "createFromFormat()" instead of the
constructor, because many date libraries don't accept a date string in the constructor. Besides, I
think createFromFormat() is better in general because it's more explicit.

For the "attribute_of" method, I made that an optional feature by introducing an interface that
defines it, which ActiveRecord\Model->assign_attribute() checks for. The reason I made this optional
is because if you use an immutable date class (e.g. PHP's DateTimeImmutable), you don't need to
worry about the dirty flagging, so there's no need to have that method.

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…tribute()

As suggested by @koenpunt. I also added createFromFormat() and format() to the interface.

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