[core] Switch update interval parameter to seconds

As correctly pointed out by @rrhuffy, restricting update intervals to
minutes is pretty arbitrary. Therefore, change logic to specify updates
intervals in *seconds*.

Also, to maintain backwards compatibility for users that already have
custom intervals in their config, allow a module to specify their own
"factor" for intervals. So a module that expects the interval to be in
minutes can set the factor to 60.

bumblebee/modules/currency.py

@@ -37,6 +37,7 @@ class Module(bumblebee.engine.Module):
             bumblebee.output.Widget(full_text=self.price)
         )
         self._data = []
+        self.interval_factor(60)
         self.interval(1)
         self._base = self.parameter("source", "GBP")
         self._symbols = self.parameter("destination", "USD,EUR").split(",")