Properly generate package via "sdist"

Include tests, configuration examples and other stuff we usually
include to the tarballs.

GitHub issue: https://github.com/OfflineIMAP/offlineimap/issues/137
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

Changelog.rst

@@ -8,6 +8,8 @@ ChangeLog
 OfflineIMAP v6.5.6.1 (YYYY-MM-DD)
 =================================
 
+* Properly generate tarball from "sdist" command (GitHub #137)
+
 * Expand environment variables in the following
   configuration items:
   - general.pythonfile;

MANIFEST.in

@@ -0,0 +1,14 @@
+global-exclude .gitignore .git *.bak *.orig *.rej
+include setup.py
+include COPYING
+include Changelog*
+include MAINTAINERS
+include MANIFEST.in
+include Makefile
+include README.md
+include offlineimap.conf*
+include offlineimap.py
+recursive-include offlineimap *.py
+recursive-include bin *
+recursive-include docs *
+recursive-include test *