Issue 59: Support crontab ranges 'x-y' and 'x/y' (#49)

* - change _range() function in sheduler:
  . add support for interval specification using "/x"
  . add support for range specification using "x-y"
  . refactor a little bit to have one block which handles a single
  item, which is used recursively when using ",", "/x" or "x-y"
# implements suggestion of issue #59
- log exception message when error occurs while analyzing the crontab
  entry

* - remove list handling of basic items in _range(), which is already
  done in the top part of the function

lib/scheduler.py

@@ -385,8 +385,8 @@ def _crontab(self, crontab):
             if not next_event:
                 next_event = self._parse_month(crontab, next_month=True)  # next month
             return next_event
-        except:
-            logger.error("Error parsing crontab: {}".format(crontab))
+        except Exception as e:
+            logger.error('Error parsing crontab "{}": {}'.format(crontab, e))
             return datetime.datetime.now(tzutc()) + dateutil.relativedelta.relativedelta(years=+10)
 
     def _parse_month(self, crontab, next_month=False):
@@ -410,7 +410,7 @@ def _parse_month(self, crontab, next_month=False):
             day_range = day_range + self._range(day, 0o1, mdays)
         else:
             day_range = self._range(day, 0o1, mdays)
-        # combine the differnt ranges
+        # combine the different ranges
         event_range = sorted([str(day) + '-' + str(hour) + '-' + str(minute) for minute in minute_range for hour in hour_range for day in day_range])
         if next_month:  # next month
             next_event = event_range[0]
@@ -507,16 +507,35 @@ def _sun(self, crontab):
     def _range(self, entry, low, high):
         result = []
         item_range = []
-        if entry == '*':
-            item_range = list(range(low, high + 1))
-        else:
+
+        # Check for multiple items and process each item recursively
+        if ',' in entry:
             for item in entry.split(','):
-                item = int(item)
+                result.extend(self._range(item, low, high))
+
+        # Check for intervals, e.g. "*/2", "9-17/2"
+        elif '/' in entry:
+             spec_range, interval = entry.split('/')
+             logger.error('Cron spec interval {} {}'.format(entry, interval))
+             result = self._range(spec_range, low, high)[::int(interval)]
+
+        # Check for numeric ranges, e.g. "9-17"
+        elif '-' in entry:
+             spec_low, spec_high = entry.split('-')
+             result = self._range('*', int(spec_low), int(spec_high))
+
+        # Process single item
+        else:
+            if entry == '*':
+                item_range = list(range(low, high + 1))
+            else:
+                item = int(entry)
                 if item > high:  # entry above range
                     item = high  # truncate value to highest possible
                 item_range.append(item)
-        for entry in item_range:
-            result.append('{:02d}'.format(entry))
+            for entry in item_range:
+                result.append('{:02d}'.format(entry))
+
         return result
 
     def _day_range(self, days):