add support of image flipping

```
To support all methods of EXIF rotation, there should be a support of image 
flipping. Please add methods Builder.flipX() and Builder.flipY(). This is also 
a prerequisite of Issue 13.

Thanks in advance.

```

Original issue reported on code.google.com by `m.zd...@gmail.com` on 16 Nov 2011 at 8:19
