[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/stock.py

@@ -29,6 +29,7 @@ class Module(bumblebee.engine.Module):
         self._symbols = self.parameter('symbols', '')
         self._change = bumblebee.util.asbool(self.parameter('change', True))
         self._value = None
+        self.interval_factor(60)
         self.interval(60)
 
     def value(self, widget):