Support for file selection (aka partial downloads)

[atomashpolskiy]

I've just added the support for partial downloads in master, so it goes into 1.7 release. There are a few issues remaining to be solved and things to be done:

• I've noticed that sometimes CLI does not terminate when all non-skipped files have been downloaded. This had never happened before, so I tend to think that I've introduced a bug somewhere. Need to investigate (might start with bt.processor.torrent.ProcessTorrentStage#doExecute, which controls when download is considered finished). Maybe it's a CLI-specific issue (it's hard to say b/c there aren't any integration tests for partial downloads yet).
• Adding a couple of unit tests for new methods in bt.data.Bitfield would be nice
• CLI client might need an additional command-line flag to disable file selection (otherwise people may be forced to keep Enter/Esc pressed for quite some time if there are many files in the torrent)
• Displaying selection prompt for an existing file (e.g. when it's not the first session for current torrent) makes little sense
• Esc keypress does not work in headless mode
• Ctrl-C keypress does not work in file selection dialog when run with GUI
• Might want to update the gif in the README to show off the new feature (looks pretty cool!)

[atomashpolskiy]

Also, current implementation does not account for some edge cases:

• Empty files are always created by bt.data.DefaultDataDescriptor, regardless of whether they've been selected or not
• Selection is performed on raw torrent file metadata (i.e. the paths from the info dictionary). Would be great to use normalized file info instead (currently normalization is done in bt.data.file.FileSystemStorage)