Epoch timestamps in RRSIG not supported

[k0ekk0ek]

Expiration time and inception time fields may be expressed as either an unsigned decimal integer indicating seconds since 1 January 1970 00:00:00 UTC, or in the form YYYYMMDDHHmmSS in UTC (see RFC4034 section 3.2).

Integer parsing code is mostly copy pasted as needed, it's probably worth consolidating and perhaps optimizing for large integer values. An initial version can be found in a comment on #25. A benchmark should tell us if it's worth it (optimization is not relevant for 0.1.0).