adds Dockerfile

This adds a Dockerfile so an environment can be built and tests run with one command. This should make troubleshooting easier, because users and developers can have a shared, known environment for comparing test failures.

Using a Dockerfile means that (with documentation and supporting scripts), only about a kilobyte is added to the repository  for all this goodness.

Running tests with the Dockerfile means that textract is completely rebuilt fresh directly from the repository each time. It does not require that any dependencies (such as tesseract, pip, antiword, etc.) be installed, nor any special Python modules.