Fix some draft cache invalidation woes.
Update Timecop and do a bundle update.
Support update: now headers on posts.

See #49.
Don't break saving just from our expectation.
Only the best bugs.

xmlschema doesn't encode fractional parts, so set_updated_time turns the
Time object into a string without fractional seconds, then post.updated
will be a Time object without a fractional part, so a comparison with t
(which does have a fractional part) fails.
Assert that we don't write out publish: now headers.

This would cause a perpetually updating post.
Add site generation test and fix the brokenness.
CHANGELOG about update: now.
Document update: now in the README.
README menu item.
Fix broken README TOC link.
Merge pull request #49 from aprescott/update-now-header

Posts should support "update: now" header