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 test case for the stage detect code
calculate md5sum from the ppm content (for now)
add xres and yres
implement copyrect and replacerect
implement threshold function
port more of ppm usage to tinycv
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
fix initial bootstrap
save pngs instead of ppm
wait for the qemu screenshot to appear - we have no other way than busy sleep ;(
Add std:: namespace in some vectors.
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.