Cancel photo download when back button is pressed

[jasonkliu]

If you select a photo list with a large number of photos, then click back the asynctask is not cancelled. Clicking on another doesn't stop the first and continues the download. Also, the progress is not shown for the new task.

[jasonkliu]

This is not (currently) an issue as it is being replaced in #43.

[mahirrana]

If you press the back button while the photos are being downloaded it does not cancel the asynctask. There are only 20 photos that are downloaded at a time and they are pretty lightweight so I didn't find this to be an issue with the Instagram feature. You guys can change this if you feel otherwise.