Changed the javascript coding style

Changed to the "sieve" style, where we first sieve out invalid inputs
and then assume that the input is correct.
Added a basic reveal mechanism using the class "reveal". We will
enhance this to allow all the specifications possible in Beamer. Note
that CSS styles defined in the stylesheet are used to configure
revealed and unrevealed elements, rather than setting it directly with
Javascript.
Merge branch 'master' of https://github.com/danielf/html-slides into danielf-master

Conflicts:
	slide.js