Creates BackgroundDownload application

Just a template app for now.
Creates UI for the app and starts to implement the FG download task

This is an incomplete commit.
Completes implementation for in-process cancellable download

Added buttons and methods for the other cases, but for now we just have
the cancellable version.
Adds the ability to perform a resumable download.

Need to find an image source that is actually resumable. At the moment
it resumes from the partial data, but it doesn't include any payload...
Completes implementation of the background download

Doesn't appear to update the app switcher, which I expected it would. But
downloading in the background now works. Just need some tidying and copy.
Slight changes to layout

Pulls cancel button apart from the start buttons
Include article write-up

This is the introduction to NSURLSession. Includes:

1) Simple download
2) Getting progress
3) Cancelling downloads
4) Resuming downloads
5) Background downloads.
Updates the code to match the new API.

Need to update the image files once flickr has come back to life.
Add the license header to the source files.
Replace images with CreativeCommons images from flickr

These will hopefully work for longer, and I know I'm allowed to use them
Merge branch 'days/01-nsurlsession'