This version includes all material that was cut from the published version in order to meet the page limit.  It is color-coded in the following way to show the differences:

dark red:       Topics not discussed in the published version.

dark green:    Extended discussion of certain topics.

dark blue:      Material that was added later.

The arXiv version will be kept in its current state (24-7-2013) apart from correcting typos.

The extended version may be updated from time to time, with new text added in dark blue.