upgrade to grails 2.2.0
change JSON to GSON and request.JSON to request.GSON

clearer what's being used & means standard converters can be used alongside new ones
tidied boolean
failing test for #7
updated readme for JSON->GSON change
javadocs
fix a couple of in-use glitches

1. Something doesn't work when using property name syntax internally in GSON
2. HttpServletRequest.reader may not work with Gson if it's forward-only