add MapSprite#_buildTileset method.
added undefined check.

tmx properties.
Update bitmap.js

引数に、x,y が足りないので追加
show dummy image at the time of image reading failure
Merge pull request #127 from fukuyama/bugfix/getBitmap

Update bitmap.js(Texture#getBitmap method)
Merge pull request #125 from fukuyama/issue/122/buildTilesets

multi tileset support.
fix
bugfix
build
update docs
bugfix