Cleanup CSS for ePub generation

[GoogleCodeExporter]

The current CSS stylesheets used contain various constructs that are not 
consistently supported in most ePub readers, to wit:

* margin: auto
* position: relative, absolute
* background-image: <url>
* float: <position>

These need to be replaced by constructs that do work even in simple ePub 
devices.

Original issue reported on code.google.com by jhellingman on 5 Jul 2010 at 12:11

[GoogleCodeExporter]

See: http://www.pgdp.net/wiki/CSS_features_not_in_EPUB

Original comment by jhellingman on 5 Jul 2010 at 12:12

[GoogleCodeExporter]

Maybe a graded approach will work best here, selecting a subset depending on 
support level.

1. Full CSS 2.0 or 2.1
2. Official ePUB limitations.
3. Bare minimum, with wide support on all readers.

Original comment by jhellingman on 5 Jul 2010 at 12:15

[GoogleCodeExporter]

Also see http://www.pgdp.net/wiki/The_Proofreader's_Guide_to_EPUB

* Remove page-numbers from generated ePub (but retain anchors)
* Somehow designate some illustrations as non-essential (decorative initials, 
page ornaments), so they can be omitted from the ePub file generated.

Original comment by jhellingman on 17 Aug 2010 at 8:54