Adding method 'restart'

Methods 'clear' and 'reset' formulated more precisely to clarify the difference to 'restart'.
FEN tag support

in 'load_pgn': FEN tag support added.
deleting move numbers even if white move is omitted.
get header part simplified allowing for leading comments and first move
number not equal 1.

method and function 'restart' added.
'update_setup' corrected.
back and next methods
Merge branch 'master' into ebrehault-back-next
Merge github.com:LotharTeichert/chess.js
Merge github.com:ebrehault/chess.js