Allow flipping images horizontally and vertically

[tibbi]

this should solve #197 . Flipping images other than 0, 90, 180 and 270 might not be perfect, but I see that those cases are quite glitchy in general.

[ArthurHub]

awesome work, thank you!

[ArthurHub]

I have added couple of thinks to your work, will publish 2.4.0 soon:

• fix flipping not working for image loaded by url (and sampled during loading)
• handle rotation by 90 degrees to change horizontal/vertical flipping, otherwise the UX is strange
• reset flip flags on reset of crop rectangle
• support flipping CropImageActivity

Thanks again.

[tibbi]

Sure, thanks