Pass around browser instance so that we no longer depend on global browser.

This is especially important when using multiple browser concurrently.

Page objects do not apply since they are only functions and have no reference
to the browser instance and thus still us the global browser.
Refactor Form.field_labels to instance property instead of a classmethod.
Refactor Form.find_widget_in_form classmethod into find_widget instance method.
Always wrap label nodes.
Remove classmethod Form.find_field_in_form in favor of instance method get_field.
Eliminate Form.find_form_element_by_label_or_name.

The classmethod Form.find_form_element_by_label_or_name is replaced with
the browser.find_form_by_field method.

The old classmethod also did not wrap the nodes properly, the one one now
always returns wrapped form node elments.
Eliminate Form.find_form_by_labels_or_names.

The classmethod Form.find_form_by_labels_or_names is replaced by the new
browser.find_form_by_fields method.
Cleanup (pep8).
Update changelog.