Added BackgroundExecutor and BackgroundTask
BackgroundExecutor hooked to Controller
Test for BackgroundExecutor
Demo to show that UI doesn't freeze while a background task is processed
AsyncExecutor use the number of cores minus 1
Added semaphore to control completePercentage lock
Added finally clause to ensure background tasks are removed from the list
Javadoc clarifying that task results listeners are invoked from UI thread
Added BackgroundExecutor as done parameter
Merge pull request #268 from anserran/background-executor

Background Executor to execute long running tasks