Get ready to add more tests for vflip/hflip
Add failing test case for in-place vflip

Thanks to @jonathantompson for spotting the bug, see issue #27
Add simpler test for vflip/hflip
Fix both test and bug
Merge pull request #29 from jucor/fix_hflip

Fix bug #27 in in-place vflip and hflip