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
PostCCO now supports :created_at attribute.

Now that we have a distinction between "new" (never persisted) and
"saved" posts (whether entities or implementation objects), the specs
needed *major* updating..

RSpec: 708 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected
Done with Issue 100 Phase 1.

Adding a Post to a Blog is no longer synonymous with "publishing" it;
publishing a post causes it to be added to its associated blog if it
hasn't yet been, but that is now decoupled from the publication state.

Also some cleanup on the PostCCO spec, now that we remember what the
correct matcher is. :anguished:

RSpec: 711 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected
Merge pull request #101 from jdickey/issue-100-stage1

Laying the groundwork for Issue 100.

RSpec: 711 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected