Add a SiteSetting to control the pop3 polling period.

The default 5 minutes may add too much lag for some sites used to mailing list performance.

Unfortunately, this seems to require restarting the server for the change to be noticed - is there any way to avoid that, or otherwise should this be noted in the setting text?
Merge pull request #2461 from peternlewis/pop3s_polling_period_mins

Add a SiteSetting to control the pop3 polling period.