Rename append() methods to add()

JsonArray and JsonObject are not just writers anymore, but simple
collection-like objects. The method name add(), known from the Java
collections framework seems like a better fit. We may introduce a
remove() method later, in this case, add() will also fit better.

Rename:
  JsonArray.append( value ) -> add( value )
  JsonObject.append( name, value ) -> add( name, value )

Fixes #4