Implement function to get all currently running lessons.

Advances BiscuIT-App-Alsijil#1.

biscuit/apps/chronos/util.py

 from datetime import datetime
 
+from .models import LessonPeriod
+
 
 def current_week():
     return int(datetime.now().strftime('%V'))
+
+
+def current_lesson_periods(when=None):
+    now = when or datetime.now()
+
+    return LessonPeriod.objects.filter(lesson__date_start__lte=now.date(),
+                                       lesson__date_end__gte=now.date(),
+                                       period__weekday=now.isoweekday(),
+                                       period__time_start__lte=now.time(),
+                                       period__time_end__gte=now.time())