Adding files/dirs in gitignore

AUTHORS ChangeLog build are created when running:
python setup.py install

But we don't want git to commit these files.

Change-Id: I670e029c1e4788d538ff8ea4d483bcd737dc10b7

.gitignore

@@ -3,3 +3,6 @@
 gertty-env
 .tox
 doc/build
+AUTHORS
+ChangeLog
+build