Added a clear() function to Model.php

Many times ``Model::create()`` is used with a single parameter of ``false``.  When this happens, what is actually happening is that the Model is being ***cleared*** rather than ***created***.

``Model::clear()`` is lightweight (since its a wrapper), provides a convenient way to clear a model, and also more intuitively describes a developer's true intentions within application code than ``Model::create(false)`` does.