Timeout in poll() function due to invalid doneChan handling

The poll() function in "cmd/account-polld/account_manager.go" always needs to provide an output to a.doneChan. Otherwise the select in Poll() does not know poll() has finished and produces a timeout.

Excerpt from the logs:

2014/10/19 17:23:15 Starting poll for account 4
2014/10/19 17:23:15 Polling account 4
2014/10/19 17:23:15 Account 4 failed to authenticate: GDBus.Error:com.google.code.AccountsSSO.SingleSignOn.Error.NoConnection: Host accounts.google.com not found
2014/10/19 17:23:45 Poll for account 4 has timed out out after 30s
2014/10/19 17:23:45 Ending poll for account 4