Add generated dates (#864)

[beckjake]

Add a field named generated_at to the root of the catalog and the manifest, which contains an RFC3339-formatted timestamp.

[cmcarthur]

you need to change one json schema. then, if the tests pass, this will be approved.

[cmcarthur]

the correct json schema draft4 type for this is:

{
    "type": "string",
    "format": "date-time"
}

[beckjake]

So, I was wondering how tests were passing after this comment - it turns out we never use this schema at all. Should I just remove it entirely?

[cmcarthur]

we need at least one json schema for the manifest, and at least one for the catalog. it's actually really important as a piece of documentation to share with the community. so ideally we'd either (a) apply it or (b) just leave it in here but do our best to correct it, even though it's not currently used. up to you which you want to do here

[beckjake]

I've changed the ParsedManifest to validate the schema (and made sure it fails on the old schema).

[cmcarthur]

hah interesting note about freezegun.freeze_time. i think this is fine as a single-use hack. let's make sure not to overuse this.

[cmcarthur]

appveyor :(

datetime.datetime(2018, 7, 24, 21, 29, 49, 490799) not greater than datetime.datetime(2018, 7, 24, 21, 29, 49, 490799)

these code changes look excellent, approved from my end. just fix that appveyor issue.