#354 fix
From now image downloading and api calls will be done in different
thread executors
- AsyncTaskEx: extracted SerialExecutor to separate class with Executor
constructor parameter
- ImageWorkder: added static field SERIAL_EXECUTOR which references to
own executor for BitmapWorker async task
- ImageWorkder: changed task.execute call to the task.executeOnExecutor
call in the loadImage method.
- PhotoEndlessAdapter: added debug output to the loadItemsGeneral method