include license and tests in source tarballs

I'd like to package itsdangerous for Fedora, but the source tarball on PyPI is missing a copy of the license. It's a requirement of the license itself that the license text be included with the source, and I need to ship it with the package as well:

https://fedoraproject.org/wiki/Packaging:LicensingGuidelines#License_Text

This patch adds a MANIFEST.in to bundle LICENSE in the source tarball.

I also added the tests, because I'd like to run them during package build to make sure everything is working.