REST Client: send asynchronous requests (#476).

Plugins providing a REST service can send the respective requests asynchronously.

BREAKING CHANGE

REST Client: the dm4c.restc.request() method has an additional "callback" parameter (the 4th parameter).

    request = function(method, uri, data, callback, headers, response_data_type)

Description of the "callback" parameter:
    Optional: the function to be called if the request is successful.
    One argument is passed: the data returned from the server.
    If not specified, the request is send synchronously.

If your plugins uses that method with more than 3 parameters -- that is if you pass "headers" or "response_data_type" you must adapt the respective calls. To keep your plugin's current request semantic -- that is **synchronous** requests -- pass undefined as the 4th parameter.
However, very few plugins should be affected.

Close ticket 476.