[BUG] Please include test suite in source release

[niol]

Describe the bug Source release does not include webapp/tests/.

Expected behavior This makes is not possible to run the test suite when building from the source tarball.

Environment (please complete the following information):

• OS flavor: all
• Graphite-web version 1.1.10
• Django/Python version all
• Setup type sources and OS packages

Additional context It would be handy to run the test suite in distributions CI infrastructures.

I'm not sure how releases are generated, but the following may be enough.

--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -8,5 +8,6 @@ include webapp/graphite/local_settings.py.example
 recursive-include distro/ *
 recursive-include webapp/graphite/ *.html
 recursive-include webapp/content/ *
+recursive-include webapp/tests/ *
 exclude webapp/graphite/local_settings.py
 exclude conf/*.conf