Add support for compressed certificates

This could be more sophisticated — it could automatically compress
certificates if they are too large, instead of expecting the user to do
so manually. But this is a good start.
Merge branch 'master' of ssh://github.com/dwmw2/yubico-piv-tool
check that stat completes correctly
cast cert_len to size_t shouldn't be negative here.

gets rid of warnings about int/size_t combinations