move (most of) ppm usage to opencv functions

The functions for opencv are stubs only (all in tinycv_impl.cc)

I removed the ppm module completely and replaced it with the cv
module. This is highly untested as testing is hard with stubs
add a destroy function, helps to surive
Add image_differ and image_avgcolor functions.
- catch errors in image_differ
- fix some crashes in the perl bindings
- save pngs instead of ppm
wait for the qemu screenshot to appear - we have no other way than busy sleep ;(
Add image_scale and image_similarity signatures.
wait for ppms to finish from kvm
Add prototypes in xs.
Add image_scale code.
Add image_similarity using PSNR.
Fix typo in documentation.
use symlinks instead of hardlinks for duplicated images - much easier to work with
remove the use of image_differ and use image_simliarity instead
Change the value for similar images
Fix width and height in scale method.
Clean image_search, using the search_TEMPLATE function
remove md5 checksums
rework the take_screenshot to not write out the png if it's the same

Just rely on symlinks
make sure we only sleep till the rest of the interval
parse the distribution needles