rework b.width for getting the width of several spreads

[ff6347]

as mentioned in #50

[trych]

Ok, I did not realize that b.width always refers to the canvas width and that it is closely related to the whole canvas mode business. I always assumed, it simply returns the width of the current page (as is stated in the reference).

I would suggest the following improvements:

1. Make b.width refer to the canvas width specifically and correct the description in the reference.
2. Make b.width work for spreads with more than 2 pages. This probably includes changing the canvas modes b.FACING_PAGES, b.FACING_MARGINS and b.FACING_BLEEDS to b.SPREAD, b.SPREAD_MARGINS and b.SPREAD_BLEEDS (or is it important that they are called Facing pages?). For backwards compatibility we could either keep b.FACING_PAGES & Co. to still work, or we keep the old names and just change how the methods work internally.
3. Introduce a new method b.pageWidth that returns the page width, independently from the current canvas.
4. Introduce a new method b.spreadWidth that returns the spread, no matter how many pages, independently from the current canvas.

[trych]

This can be closed, as width now returns the correct value referring to the currently set canvas mode. My suggestions about pageWidth and spreadWidth from above could rather be solved via bounds() (see #276).