Ship extra bits with the pypi tarball.

This change should make new tarballs uploaded to pypi include various
nice pieces:

- The README
- The LICENSE
- The tests

The text of the license actually specifies that the full text must be
distributed with all copies of the software.  So, you need it to be in
compliance with the MIT license.

The README is just nice to have, and the tests are particularly nice for
my use case.  I am packaging vcrpy for inclusion in the Fedora linux
distribution and:

- We like to use the tarball from pypi because it is the same source
  distribution that everyone else is using.
- We like to run the tests before we build the rpm in our build system
  to make sure nothing crazy is going on.

Of course, we can use the tarball for the source and then do a second
step to clone the source and get the tests.  But, this is more work than
we like if we can just get the tests added to the tarball.  Other
distributions (like Debian) like this too.

MANIFEST.in

@@ -0,0 +1,4 @@
+include README.md
+include LICENSE.txt
+include tox.ini
+recursive-include tests *