Include all the source files in MANIFEST.in

Because otherwise doc/INSTALL.rst lies to users when it tells them the
source tarball will have a buildout.cfg and everything.  Lying to users
is Bad.

(I don't know what to do about the .mo files that are in git.  Should
they be included in the sdist?)

MANIFEST.in

@@ -7,3 +7,9 @@ global-exclude *.dll
 global-exclude *.pyc
 global-exclude *.pyo
 global-exclude *.so
+
+include *.py
+include .travis.yml
+include buildout.cfg
+include sources.cfg
+include versions.cfg