adds README.md
changes bot names to Stockfish N
adds game in progress check to Bot move function

hopefully this will fix the problem with stockfish processes hanging
around - if you get the engine to move in a checkmate position, it
says bestmove (none) which wouldn't match the regex.
changes Bot to spawn an engine in the constructor and never kill it

hopefully this will mean that there are only ever as many stockfish
processes as bots - the previous fix attempt (not trying to move in
checkmate) didn't work.
moves engine setup code below required variable definition in Bot constructor
adds newline to stockfish go command in Bot
checks for existing game before playing rematch in Bot
adds return true to Bot acceptSeek some() callback
sets seek to null on Matched in Bot
updates User to send /open_seek/new instead of /open_seeks
sets increment to always 0 for stockfish and adds some longer time controls in Bot
checks current game isInProgress before abandoning rematch in Bot
implements rematches properly in Bot - offer, wait a bit and then cancel and set game to null
removes _waitingForRematch checks in Bot - not used