DataProviders should have a common lifecycle

[jdjkelly]

I want authenticated API sources flow through the Warden + Devise + Omniauth provider. They should have a common lifecycle.

On initial authentication, all historical data should be downloaded from the provider (if possible). Then, a synced_at attribute should be set with the current time stamp.

Data should be stored with any uniquely identifiable information from the API. This data should be indexed.

Subsequent requests should:

• only request data after the synced_at date
• use the stored ids as a uniqueness constraint
• be performed without the need of scheduling (assuming they aren't web-hook style)

[jdjkelly]

Progress made in these PRs:

#20
#22