add the same warnings package as yubico-c
refactor to use chalresp function from ykpers

so challenge_response() now calls yk_challenge_response() to
do the yubikey internal stuff.
use pbkdf2 to process the exepected response

this bumps the version on the state file to 2
old files can still be read but new files will use the new format
fix warnings
fixup warnings
fix warnings
ignore signed releases
add -i switch for setting iterations with ykpamcfg