sync menu breaks darwin (probably also windows) syncdaemon

[Mike McCracken]

syncdaemon's start.py calls start_timer() on UbuntuOneSyncMenu().
windows and darwin use a dummy no-op class in sync_menu/common.py, which has no attribute 'start_timer'.

Traceback (most recent call last):
  File "twisted/internet/defer.pyc", line 545, in _runCallbacks
  File "/Users/mmccrack/Documents/Canonical/Source/test-improve-buildout/scripts/dist/build_to_sign/UbuntuOne.app/Contents/Resources/UbuntuOne Syncdaemon.app/Contents/Resources/ubuntuone-syncdaemon.py", line 90, in check_death
  File "twisted/python/failure.pyc", line 346, in trap
Failure: [Failure instance: Traceback: <type 'exceptions.AttributeError'>: 'UbuntuOneSyncMenu' object has no attribute 'start_timer'
twisted/internet/defer.pyc:362:callback
twisted/internet/defer.pyc:458:_startRunCallbacks
twisted/internet/defer.pyc:545:_runCallbacks
twisted/internet/defer.pyc:1095:gotResult
--- <exception caught here> ---
twisted/internet/defer.pyc:1039:_inlineCallbacks
/Users/mmccrack/Documents/Canonical/Source/test-improve-buildout/scripts/dist/build_to_sign/UbuntuOne.app/Contents/Resources/UbuntuOne Syncdaemon.app/Contents/Resources/ubuntuone-syncdaemon.py:236:async_main
ubuntuone/syncdaemon/main.pyc:163:__init__
ubuntuone/syncdaemon/main.pyc:168:start_sync_menu