ppm: fix header parsing

According to the spec[1] there is a _single_ whitespace character right
after the max color value. So there is no need to skip characters after
the last call to `ppm_get_long`.

[1]: http://netpbm.sourceforge.net/doc/ppm.html
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
Merge pull request #26 from deltheil/master

ppm: fix header parsing