add AI.
Merge pull request #6 from calfjohn/master

2 commits
replace new pics and add itemcontroller.
Merge branch 'master' of github.com:fusijie/LudumDare31
Merge branch 'master' of github.com:fusijie/LudumDare31
Merge pull request #7 from calfjohn/master

2 commits
Merge branch 'master' of github.com:fusijie/LudumDare31
nothing.
Merge branch 'master' of github.com:fusijie/LudumDare31
Merge branch 'master' of github.com:fusijie/LudumDare31
move bullets position at tower