Simplify requirements for enabling autoRefreshFromNetwork

[blakewatters]

When autoRefreshFromNetwork is set to YES the autoRefreshRate must also be set in order for the autoRefresh to work. I think this can be simplified by only setting the autoRefreshRate. So by default autoRefreshRate = 0 and any non zero rate would turn autoRefreshFromNetwork on.

A minor point on the variable name: since the autoRefreshRate is of type NSTimeInterval perhaps a more congruous name for the variable could be "autoRefreshInterval".