fix bug when iterating over all existing posts in a thread (#23)

* fix get posts

* remove tail/head and fix posts command

rfd/utils.py

@@ -0,0 +1,8 @@
+"""This module provides utility functions that are used within rfd"""
+
+def is_int(number):
+    try:
+        int(number)
+        return True
+    except ValueError:
+        return False