Revert "refine background to fit 720p"

This reverts commit e87afab7fda6957d9660b35cd561fb99723c1ab9.
Revert "try a new background"

This reverts commit b2107f0640d10e5bf8512afab9f115082ccffabc.
hide Base into Board from other objects

some fixes:
1. for killing pawn
2. make variable as local var for Field.prototype.getPawns
   and Field.prototype.kill
refine framework of the game
add support for disconnection