vera improvements

Improvements to the implementation of [vera].

* [x] Make `vera` implementation completely independent from `annotate`.  While vera's ERAV is logically an extension of annotate's EAV, sharing the same *AV models makes the whole system more fragile.
* [x] Denormalized `EventResult` model for fast read-only access
* [x] Backend charting service for [wq/chart.js]
* [x] Tests for `vera`, `dbio`, and `annotate`

[vera]: http://wq.io/vera
[wq/chart.js]: http://wq.io/docs/chart-js