Added set_machine.py.

Corrected usage messages to include dibond.

set_machine.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python
+
+from __future__ import print_function
+
+import sys
+
+def set_machine(machine):
+    text = "include <%s_config.scad>\n" % machine;
+    f = open("scad/conf/machine.scad","rt")
+    line = f.read()
+    f.close()
+    if line != text:
+        f = open("scad/conf/machine.scad","wt")
+        f. write(text);
+        f.close()
+
+if __name__ == '__main__':
+    args = len(sys.argv)
+    if args == 2:
+       set_machine(sys.argv[1])
+    else:
+        print("usage: set_machine dibond|mendel|sturdy|huxley|your_machine")
+        sys.exit(1)