Add Flip and Rotate

Mikera,

Thanks for this great library - it really makes basic image processing a breeze. I added a rotate and flip function based on your resize function using Scaler. Hope it might be useful.  

One issue: although the code works and the tests pass it does trigger the following warning:
`Reflection warning, mikera/image/test_core.clj:35:15 - reference to field getWidth can't be resolved.`

Thanks
zach cp