Workaround for #4

[giovanniberti]

Added check for old lastUpdate value and deferred page refresh to compensate for the time the server takes to analyze the pages and to update dashboarddata.json

It does not really fix #4 because sometimes the page does a double-refresh (and flickers in doing so). The dashboard doesn't show negative refresh times anymore, however.

[giovanniberti]

In fact it does check if the value is the old one. It's only a "visual" problem now (the flickering), which can be solved, for example, with XHRs instead of a full reload. I don't think we have any other options, because we don't know when (or if, in the case of rate limiting), the server is going to update the data.

[nmaggioni]