When parsing trackers, revert to the old-style 'announce' URL if 'announce-list' is corrupt

I've seen one torrent which has an empty 'announce-list', causing it to be rejected completely. This may be overly strict, even though the presence of of an empty 'announce-list' should be considered a bug.

Construct the tracker talker before hashing pieces

As constructing the tracker talker may fail (due to missing/corrupt metadata), we must do so before scheduling pieces since an exception will cause our object construction to be aborted - yet the hasher won't know about this and invoke callbacks in an object already gone.

Note that the tracker won't actually be called during initialization (Torrent::heartbeat() talks to it) so this change does not introduce races.

Rework the client so that it caches most of the torrent metadata and queries it less

This still needs work, but this is a nice start

Return a const reference to the torrent name instead of a copy

Introduce per-torrent user pointers

Users should mostly just derive from the Torrent class, but having an own pointer is easier for now. These are initially NULL so that callbacks can detect whether they aren't being added just yet.

Avoid code duplication by using registerPeer()

Note that this function will now be called before the peer ID is known; this shouldn't be a problem.

Add a 'torrent added' callback

This is intended to let the client update its administration so that it doesn't have to keep on asking the Overseer for torrent lists.

Do note that callbacks *may* be called before this one is - this mainly happens when hashing is being performed.

Use local dummy callbacks when none are provided, and never ever clean them up

This makes it possible to use derrived objects for callbacks

Plug a memory leak

Introduce the Tortilla namespace