Bug 902999 - [Clock] Add Time To In-Progress Timer, "+1 Min"

https://bugzilla.mozilla.org/show_bug.cgi?id=902999

- Moves value accessor property to Picker.prototype
    - Defines set accessor operation for Picker.prototype.value
    - Defensive accessor handling

- Defines Timer.prototype.plus(seconds)
    - Add an arbitrary number of seconds to the current timer.
    - Seconds are auto-converted to milliseconds
    - duration and endAt are extended accordingly

- Tests for
    - Picker.prototype.value
    - Timer.prototype.plus()

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>