Add gradlew and fix potential IllegalArgumentException

Fixes potential IllegalArgumentException:

java.lang.IllegalArgumentException: width and height must be > 0
       at android.graphics.Bitmap.createBitmap(Bitmap.java:1151)
       at android.graphics.Bitmap.createBitmap(Bitmap.java:1130)
       at android.graphics.Bitmap.createBitmap(Bitmap.java:1097)
       at com.github.amlcurran.showcaseview.ShowcaseView.updateBitmap(ShowcaseView.java:169)
       at com.github.amlcurran.showcaseview.ShowcaseView.access$100(ShowcaseView.java:29)
       at com.github.amlcurran.showcaseview.ShowcaseView$1.run(ShowcaseView.java:148)
       at android.os.Handler.handleCallback(Handler.java:733)
       at android.os.Handler.dispatchMessage(Handler.java:95)
       at android.os.Looper.loop(Looper.java:157)
       at android.app.ActivityThread.main(ActivityThread.java:5872)
       at java.lang.reflect.Method.invokeNative(Method.java)
       at java.lang.reflect.Method.invoke(Method.java:515)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:852)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:668)
       at dalvik.system.NativeStart.main(NativeStart.java)