Added support for iso dates auto conversion on $fetch and $save

1. When your model has a property with ``Date`` object, before it gets synced it will be converted to ``ISO-8601`` string. After the model is saved, the properly will be converted back to ``Date`` object.

2. When your backend model returns a property with ``ISO-8601`` compatible string, after compiling a object it will get converted to ``Date`` object.

This options is **disabled** by default and requires user to set ``$autoConvertDates`` to ``true``. User can manually convert a chosen property using ``$isoToDate()`` and ``$dateToIso()`` functions. It accepts a single property name, an array of property names or an empty array which will apply the function to all available and compatible properties of the model.

This functionality does not support deep conversion (only first level of properties are handled and no array of dates are supported - maybe wanna write it?).

This PR is not aware of #2 changes (they are independent).