refactor(client): Add a timer-mixin that takes care of setTimeout housekeeping.
Convert modules to use timer-mixin.setTimeout
Move timer-mixin into views/mixins subdirectory.
Cleanup based on @zaach's suggestions.

* Save `this._workingTimout` when setting the working timeout!
* Wrap setTimeout tests in a `TestHelpers.wrapAssertion` to ensure tests complete even if they fail.
Merge pull request #1286 from mozilla/issue-1010-setTimeout

automate setTimeout housekeeping