init checkin of a ticker attribute for input plugins
updated stat_accum_input tests to use the new input plugin ticker
removed tickChan from stat_accum_input
updated docs to reflect the use of ticker_interval and that it is now
a required configuration parameter
revised as per code review

- added TickerInterval attribute to the stat_accum_input  with a default of 10 seconds
- switched the access to Ticker() to use a local instance for mocking purposes
Merge pull request #278 from mozilla-services/features/issue_259

Features/issue 259