Only pop the theme if the slideconf specified one.

Previously you could get into a state where all the themes (including
the base specified in conf.py) could be popped off, leaving Sphinx
unable to find the templates. This usually happened when you were
using ``slideconf`` to disable autoslides without overriding the
default theme.
Added support for marking sections as slides when autoslides is off.
Correctly fix up relative image paths for files in subdirectories.
Added support for inline-contents on Slide nodes.
Updated NEWS.txt to reflect changes since 0.5.