Widgetastic.core

Adding FillStrategy enhancement from izapolsk that allows for more fine-grained control of `View.fill` without consumers having to re-implement it.

Includes restoration of the 'exception' kwarg in wait_for_element, with the default behavior of the function unmodified. This is in order to support some implementation styles using this browser plugin method.

Modifications to `Browser.wait_for_element` that remove the ambiguous 'exception' kwarg, and modify method behavior to be more predictable as a 'wait' type method.

This release contains 2 major changes:
1. widget.py has been turned into package
2. Table widget supports rowspan/colspan attributes