oops. include preaggregation table in truncation list.
Bug Fix: run compute before checking variance equality.
TimerRollup.toString() still reflected that it extended BasicRollup.
Reader/Writer support for preaggregated metrics.

* Expose the static `ColumnFamily` instances in AstyanaxIO. They get passed around a lot.
* Introduce `IMetric` interface that can be used pretty much everywhere we use `Metric` references.
* `AstyanaxReader` gets` `getDataToRoll`, which aims to be type ignorant (could use it on any rollup type including full resolution metrics). It is demonstrated in `getTimerDataToRoll()` and could probably used in the other metric reader methods. Future PR maybe.
* `AstyanaxWriter` gets a generic `insertMetrics()`, which could be used as a basis for all the metric insertion methods.  Demonstrated by replacing all uses of `insertPreaggregate()` with it.
* Some tests.