use proguard tool to reduce size of uberjar

Add two tasks named with proguard and proguardCheck.Task proguard uses proguard tool to shrinking the uberjar.The size is reduced from 8M to 4.6M.

Of course here are more thing we can do to make a smaller jar and proguardCheck is a way to run test with the package jar.Be relax to change proguard.pro settings and use proguardCheck to check it works well or not.

Just as you say,gradle is more flexible and easy to do custom task(^_^)