Add gitignore for nanoc

Add:
 * General nanoc gitignore file

nanoc.gitignore

@@ -0,0 +1,7 @@
+# For projects using nanoc (http://nanoc.stoneship.org/)
+
+# Default location for output, needs to match output_dir's value found in config.yaml
+output/
+
+# Temporary file directory
+tmp/