Move safeDiv to Util
Include rate in counter rollups.

* CounterRollup was no longer a single value, which means SingleValueRollup and its serializer no longer had a purpose. Gone.
* Noticed that set rollups are the same as BasicRollup. Cleaned things up a bit.
get rid of numSamples in counters.
SingleValueRollupTest -> CounterRollupTest
include rate in unit tests for counter.
use the right constant for set rollup tests.
address PR comments. fix spacing, introduce constant, change order of comparisons in .equals.
Merge pull request #183 from gdusbabek/counter_rates

Include rates with counters.