Create a default module to use as a sentinel value

This looks better in the documentation than utils.Default,
cleans the utils with specific stuff like this, and users
may use it more easily.

telethon/default.py

@@ -0,0 +1,5 @@
+"""
+Sentinel module to signify that a parameter should use its default value.
+
+Useful when the default value or ``None`` are both valid options.
+"""