Add [ut_pex] support
Remove from [connecting] set when [connect] fails
Fix bug in responding to block requests

We were using the old piece/ofs/len scheme instead of piece/block.
PEX: fix bug (6 bytes are 8 * 6 bits not 8 bits)
Update README
PeerMgr: no outgoing peers after completion
Rechoke when unchoked peers leave, change interest
Ignore snubbed peers in choke algorithm
Simplify [Choker.unchoke_peers]
Close peer sockets after we are done using them
Initialize Store with metadata on creation
Send BITFIELD instead of a bunch of HAVEs
Choose block size so that piece size is a multiple