Rename getValue() to get()

JsonArray.getValue( int ) -> get( int )
JsonObject.getValue( String ) -> get( String )

Renamed because:
a) it's shorter
b) it matches the get methods of the Collections API (Map, List)
Enable chaining of append methods

Allow for appending multiple elements with less code, e.g:

    array = new JsonArray.append( 23 ).append( 42 ).append( true );

Fixes #5