Torrent processing chain

[atomashpolskiy]

Need to introduce a notion of torrent processing chain with clearly separated stages: metadata retrieval, torrent parsing, download, finalization. Library client will be able to register listeners that will receive the results of a particular stage. For instance, this might make it possible for the user to select specific files that should be downloaded from a multi-file torrent. Another example would be to move/rename the temporary download files (if we decide to add such option) after the download is complete.

[atomashpolskiy]

For 1.3 this will be constrained to just introducing notion of processing chain and stages and refactoring internal code accordingly. Listeners will come in later versions.