Include assets in the source tarball. - #743

Related: we ought to change travis to run unit tests against an installed version of mypy. Probably should switch to setuptools though.

@fladi Thanks for the PR! Should we also include stubs (everything under stubs/)?

@o11c Not sure about that, as at least I almost always run unit tests in a local clone of the repo. However, we should verify that we can run the installed version of mypy.

Hmm I already forgot what's the purpose of MANIFEST.in. It took me a while to figure out the last time because the name isn't very googleable. Anybody have context on this? The commit message should be more explicit about what this is used for.

Closing as I don't have enough context to review this. Feel free to resend this PR with more context.