Rework the primary interface -> single method

This reworks the primary interface to the Ostend module so that it
maintains a single method interface.

However, there are now much more control over how the module functions.
This control is exerted by the following instance variables:
* @ostend_attr_type -> this controls the type of attr created
    * valid values - any of :accessor, :reader, :writer
* @ostend_attr_filter -> a whitelist array of allowed attributes
    * note that if this is set, but ostend_strict is not, Ostend will happily and **silently** discard the hash keys that are not included
* @ostend_strict -> t/f to decide if you should throw an error when a user attempts to assign a disallowed attribute