Support for private columns and filtered objects

[svenbw]

When one wants to send an ORM object trough an API some columns should be remain private or hidden. This can be done using the $_private variable in the model.
Calls like as_array() will return only the non private column values.

An extra function as_object() is also added this will return a new object instance with the column values, it differs with the casted array ((object)$some_model->as_array()) that each value will be returned as it's native type (string, int or float).

If the functions are called with the TRUE argument (as_array(TRUE) or as_object(TRUE)) all column values will be returned.
Also leaving $_private empty will return all column values which assures backwards compatibility.

[enov]

@svenbw thank you for your pull request, it's very comprehensive.

Unfortunately, most of the active maintainers are probably not using this module. That's why PRs are sitting around here for a while until someone merges them. Sorry :(

[svenbw]

@enov in the openclassifieds2 project the ORM module is still used by @neo22s, so maybe there is still some hope :-)

[svenbw]

Added behaviors for ORM.
This commit adds behaviors to the ORM model; it is inspired by the behaviors in Laravel and allow a model to write/modify data during the creation, updating or creation of the model.
A good example is a slug for a page, this is based on a title and needs to be updated during the commit.
It is possible to update data in a model creating a subclass of the model (e.g. ORM_Slugged) but if there is the need to update another column one needs to create another subclass.

With behaviors one can add a slug, guid, creation timestamp, ... by defining the behavior() function in the model. This function returns an array just like rules and filters.
I've included a guid and slug behavior in the default ORM module.