Post#created_at attribute added.

This was a more pitched battle than it "should" have been, thanks to
discoveries made re FactoryGirl::Rails (see the updated `PostData`
factory). Also fixed a couple of affected files where the (generally
useless) `Object#clone` method had been used (which creates a shallow
copy of an object instead of the implied deep one); `Marshal.load` when
passed the result of a `Marshal.dump` call will create a "true",
distinct clone.

RSpec: 705 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected