Include tests in source distribution (#514)

diff --git a/MANIFEST.in b/MANIFEST.in
index cc692b3..dc06524 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -4,3 +4,5 @@
 include README
 include FAQ
 include setpath.sh
+recursive-include tests *
+global-exclude *.pyc __pycache__