Scaling and shebang

Fixed a critical bug in --prod argument. Was referencing wrong variable.
Added shebang for the script. Only need to set the script to runnable (chmod +x) and it can be run as normal. Saw a different shebang in one of the other pull requests but this one is better as it is environment independent.
```
./android-res-resize.py
```

Also added a simple scaling option, doesn't get more simple than that.

Refers to #6 and #7