deprecated JsPath.readOpt/writeOpt/formatOpt for readNullable/writeNullable/formatNullable + added JsValue.transform + doc
renamed ConstraintsWrites/Reads/Format.nullable to optionWithNull to prevent confusion with read/write/formatNullable + added JsPath \ running through JsArray
added JsResult.recoverTotal besides recover and a few corrections
minor correction in doc
Merge pull request #662 from mandubian/json-rc2

Json refact : read/write/formatOpt deprecated -> read/write/formatNullable