Get rid of vestigial 'FlushInterval' config option that has been supplanted by
TickerInterval, and add source docs for the StatAccumInputConfig.
Merge branch 'dev' into features/remove-stataccum-flushinterval

Conflicts:
	pipeline/stat_accum_input.go
Merge pull request #302 from mozilla-services/features/remove-stataccum-flushinterval

Get rid of vestigial 'FlushInterval' config option that has been supplan...