[modules/various] Use new interval mechanism

Use generic interval mechanism in most of the modules that use slow
updates.

Only exception: getcrypto, as the interval is specified in seconds there
and I want to retain backwards-compatibility.

fixes #220

bumblebee/modules/stock.py

@@ -31,6 +31,7 @@ class Module(bumblebee.engine.Module):
         self._currencies = self.parameter('currencies', None)
         self._baseurl = 'http://download.finance.yahoo.com/d/quotes.csv'
         self._value = self.fetch()
+        self.interval(60)
 
         if not self._currencies:
             self._currencies = '$' * len(self._symbols)