[Android] Create our own download-manager

[SimonSimCity]

Sooner or later, I guess we have to maintain our own version of a download-manager. #33 and #29 now more and more show the need for a self-maintained downloading service on Android.

[SimonSimCity]

Even if this is a java-example, I guess it could be a good starting point: http://stackoverflow.com/questions/3028306/download-a-file-with-android-and-showing-the-progress-in-a-progressdialog