Make not_before and not_after be integers.

I'm no longer storing the raw ASN1_TIME structures for later parsing, so
nuke everything related to that. This also means we can go back to returning
void for pe_parse_certificates.

ASN1_GetTimeT() is a cleaned up and slightly modified version from
http://stackoverflow.com/questions/10975542/asn1-time-conversion. It seems
to work well enough, but I suspect it will have some edge cases.