handle case where parsedTorrent.length is undefined
handle case where parsedTorrent.length is undefined

- Expose `torrentLength` so the user can set it when they know the
torrent length. (With a magnet uri, they won’t know the length at the
time the Client is instantiated)
- UDP Client: Send FFFFFFFFFFFFFFFF for ‘left’ param when we don’t know
the size. (This is what Transmission does)

Fixes #15.

precisit/bittorrent-tracker,prozacchiwawa/bittorrent-tracker,maxogden/bittorrent-tracker,jhiesey/bittorrent-tracker,feross/bittorrent-tracker,LinusU/bittorrent-tracker,feross/bittorrent-tracker,jhiesey/bittorrent-tracker