minor tweaks

Clarifying some (random) things that are incomprehensible to english
speakers who don't live in an American context, or otherwise seem
unnecessarily cryptic
typo

Thanks M. Corlosquet
inital version, clone from rvguha
added vehicles extension proposal without examples
fixed about vs resource in rdfa
https://schema.org/ -> http://schema.org/ in 21 places
added <br /> to longer textual definitions (also for GoodRelations elements)
replaced schema:XYZ by local name plus relative link.
added acknowledgments section
added TwoStrokeMixture
fixed label for specialUsage property
finalized Microdata and RDFa examples
removed DBPedia and Freebase from proposal and examples
reorganized files, changed GAE appid
fixed superclass for Car
removed vehicles part from schema.rdfa (now only in sdo-automobile.rdfa)
fixed superclass for Car in schema.rdfa and sdo-vehicle-schema.rdfa
fixed bug in microdata example for Car
fixed bug in rdfa example for Car
minor