Periodically update books

Fixes #21

fedireads/routine_book_tasks.py

@@ -0,0 +1,16 @@
+''' Routine tasks for keeping your library tidy '''
+from datetime import datetime, timedelta
+from fedireads import books_manager
+from fedireads import models
+
+def sync_book_data():
+    ''' update books with any changes to their canonical source '''
+    expiry = datetime.now() - timedelta(days=1)
+    books = models.Book.objects.filter(
+        sync=True,
+        last_sync_date__lte=expiry
+    ).all()
+    for book in books:
+        # TODO: create background tasks
+        books_manager.update_book(book)
+