Create common package

Moves cryptographer to common. Also adds constants and defines the leghtn of the locator to avoid hardcoding it in almost every file

common/constants.py

@@ -0,0 +1,8 @@
+# Locator
+LOCATOR_LEN_HEX = 32
+LOCATOR_LEN_BYTES = LOCATOR_LEN_HEX // 2
+
+# HTTP
+HTTP_OK = 200
+HTTP_BAD_REQUEST = 400
+HTTP_SERVICE_UNAVAILABLE = 503