Problems about JSONDownloadTask

[pangeneral]

The inner class JSONDownloadTask defined in

1. be.brunoparmentier.openbikesharing.app.activities.BikeNetworksListActivity
2. be.brunoparmentier.openbikesharing.app.activities.StationsListActivity
   holds strong reference to the Activity which can lead to memory leak.
   I think we should change JSONDownloadTask to an static class or outter class.

Besides, the instance of JSONDownloadTask is not cancelled, which can lead to result loss. I think the instance of JSONDownloadTask should be cancelled before the Activity is destroyed.