Removed the double-RRSIG ZSK Rollover

This has all the disadvantages of the Double-Signature and
Pre-Publication methods, and none of the advantages of either. It
is extremely unlikely to be used in real life, so a detailed
timeline is of no practical use.
Remove the end of zone signing as a specific event on the timeline

This backs out a change made in an earlier revision of the -04 draft
after misgivings from Johan Ihren and Matthijs Mekking.
Corrected equation in event 7 of Pre-Publication ZSK Rollover

The equation was: Tret(key) >= Tact(key) + Lzsk.  The ">=" was
replaced by "=".  In this type of rollover, a key becomes retired
as soon as the successor key is made active.  Although the lifetime
of a ZSK is set by policy, Lzsk is the actual lifetime of the key,
the interval between its activation time and the activation time
of the successor.
Expanded text about the "retired" state in the double-signature ZSK rollover

Unlike the pre-publication method, a key in this rollover is still
used to sign RRs during its retirement period.
Merge pull request #6 from samorris/master

ZSK Rollover Changes - 9 May 2014