Unbreak master page rendering in ODPs.

Restricted the `z-index: 1` hack for draw|frame to
frames within office|text, since it was intended for
ODTs in the first place.
Merge pull request #531 from adityab/zframe

Unbreak master page rendering in ODPs.
Make html files of editor and viewer HTML5 validated

Check done with http://validator.w3.org/check
Merge pull request #528 from kossebau/validatedHTML

Make html files of editor and viewer HTML5 validated
Add section "Creating webodf.js" to README.md
Do not duplicate the README in the error messages