Fix TimeWindow not resetting buckets when re-using them.

Fixes issue #13

Support for custom timestamp values

AppendWithTimestamp func to support custom timestamp values

Migrate to go modules

Merge pull request #6 from asecurityteam/go-mod-migration

Migrating repo from dep to go modules

Fix bug with empty percentiles

Merge pull request #5 from asecurityteam/fix-empty-percentiles

Fix percentile reduction when there are no values

Add tests for reductions

Add tests for Count, Sum, and Avg in reduce.go.

Fix for min/max reductions

Merge pull request #3 from faycheng/bug-fix/reduce

bugfix: max & min reduction func