added substep functionality

add class 'substep' to any element in a 'step'. Like the normal steps
substeps will be in on of the three temporal states 'future', 'present'
or 'past'.
If a data-group is specified all substeps in this group are shown at the
same time.
data-resetsubsteps="false" let all substeps past when you reenter a step
added graphical assets

added new class "asset" to steps with data-showafter and data-hideafter.

assets are just for background graphics and are not part of the
presentation steps.
the data attributes can contain multiple step ids seperated with a space
(' ')
added data-next and data-prev for nonlinear presentations

adding data-next and data-prev will specify the next step to show when
you leave the current step in this direction. The data elements can
contain the id or the index for the next step.
merge with assets
Merge branch 'nonlinear' into no0x9d-extensions

Conflicts:
	js/impress.js
added missing '{' after manual merge