Add create and update timestamps to Resource objects

[aaron-junot]

This addresses #66.

I am thinking we'll probably need to add a parameter to the GET /resources route so that the front end can actually query for resources based on the created_at or last_updated data. Let me know if you want to see that as part of this PR.

I'm willing to have a discussion about whether we should include these timestamps in the data returned by all requests (I wasn't sure so I decided to not include it until we can have a discussion about it).

[aaron-junot]

Query should look like this GET /resources?updated_after=2018-11-24

The model itself should have something like this

created_at = Column(DateTime(timezone=True), server_default=func.now())
last_updated = Column(DateTime(timezone=True), onupdate=func.now())

That way, any changes made on the database (outside the app) will also have timestamps. See here

[aaron-junot]

@wimo7083 I need help writing tests for this. Can you hit me up on slack when you get a chance?

[apex-omontgomery]

Very nice, looking great.