Handle KeyboardInterrupt in buck.py.

Summary:
Terminating a buck build with CTRL-C sometimes results in a Python
Traceback being shown on the terminal.

Fix this by catching KeyboardInterrupt in the main() method. Don't
do anything with the caught exception; just pass.

Test Plan: Build something. Terminate it with CTRL-C. Don't see a TraceBack.

src/com/facebook/buck/parser/buck.py

diff --git a/src/com/facebook/buck/parser/buck.py b/src/com/facebook/buck/parser/buck.py
index 44e9f3c..46b8719 100644
--- a/src/com/facebook/buck/parser/buck.py
+++ b/src/com/facebook/buck/parser/buck.py
@@ -866,4 +866,7 @@
 
 
 if __name__ == '__main__':
-  main()
+  try:
+    main()
+  except KeyboardInterrupt:
+    print >> sys.stderr, "Killed by User"