prepend DKIM-Signature, as recommended by RFC
simplify ordered hash implementation of DkimHeader
test for Dkim 'h=' tag in tests
Sign headers in the order they appear

There is no serious technical reason to enforce this order. However,
both gmail and RFC 6376 use this order. Downside is that this becomes
slightly awkward when there are multiple occurrences of a header field.
make Dkim::Interceptor prepend header as well
move header parsing into Header.parse
add canonicalized headers
remove HeaderList
missing require
extract tag-value list from DkimHeader
add encodings for tag-value lists
replace .ord with .unpack('C') for 1.8
move encoding from SignedMail to DkimHeader
ensure time does not change in interceptor test
fix for CanonicalizedHeaders

* Fix handling of missing headers (which should be skipped)
* faster, 4 headers should be plenty
* test repeated and missing headers
modify $LOAD_PATH in test helper
should be specifying 1.0.0.pre version
update changelog