For the future: dedicated module for NAIF names

Since this cache of dictionaries will be needed by users both of
`jpllib.py` and of `planetarylib.py`, it should have its own home.
Tests and documentation?  Someday!

skyfield/naifcodes.py

@@ -0,0 +1,16 @@
+from jplephem.names import (
+    target_name_pairs as code_name_pairs,
+    target_names as code_names
+)
+
+name_codes = dict((name, code) for code, name in code_name_pairs)
+
+def numbered_name_of(code):
+    """Given a code, return a string giving both the code and name.
+
+    >>> numbered_name_of(301)
+    '301 Moon'
+
+    """
+    name = code_names.get(code, '(Unnamed)')
+    return '{0} {1}'.format(code, name)