1. 修复按DEL崩溃问题
2. 修改背景颜色
提交bin文件夹
Merge branch 'master' of github.com:blueolive/Simple-Calculator
Merge branch 'master' of https://github.com/blueolive/Simple-Calculator

Conflicts:
	Calculator/src/mengwu/project/calculator/CalculatorActivity.java