Captcha Notification Service

Added (optional) Notification Service to check for new Captchas in Background even when screen is turned off.
This is done by using the AlarmManager, which starts the Service repeatedly (using the interval from the preferences) when leaving the App.
The Service can be enabled/disabled very easily with a checkbox in the menu.

The implementation is done in a pretty simple way. So maybe it could be done in a more efficient way by using the background thread which holds the connection? As far as I know the Threads won't be alive for a long time when the screen was turned off.