README, CHANGES: .txt -> .rst

So that both readme and changelog automatically render well on

https://lab.nexedi.com/nexedi/cloudooo  and on  
https://github.com/Nexedi/cloudooo

CHANGES is also renamed -> CHANGELOG along the way, because that's what
gitlab autodetects as changelog file name:

https://lab.nexedi.com/nexedi/gitlab-ce/blob/827d3914/app/models/repository.rb#L353

/cc @kazuhiko, @gabriel

/reviewed-on !3

MANIFEST.in

-include README.txt
-include CHANGES.txt
+include README.rst
+include CHANGELOG.rst
 recursive-include cloudooo *

setup.py

@@ -6,7 +6,7 @@ version = '1.2.5-dev'
 def read(name):
     return open(name).read()
 
-long_description = (read('README.txt') + '\n' + read('CHANGES.txt'))
+long_description = (read('README.rst') + '\n' + read('CHANGELOG.rst'))
 
 install_requires = [
           # -*- Extra requirements: -*-