Use poetry

[JJMinton]

This PR is slightly opinionated and contains a number of different changes, but they're all things I did a part of getting the repo installed and tests running (although still not passing as per #40). I'm happy to rework it to include/exclude any of the elements or break it up into more focused changes.

The contributions are:

• Use poetry: this is a newer dependency management solution with growing popularity. It's more declarative than setuptools, which I find easier to use, It also locks dependencies, which improves reproducibility between contributors.
• add a .gitignore file for convenience while developing.
• Package
  • Update pyemd as this was failing to build with a newer version of cython
  • Add missing dependencies
  • Add pyrouge as an extra
  • Add pytest as a development dependency. Pytest captures stdout and stderr output while running tests which means the noise produced by downloading/loading models and data from various packages is presented in a more readable format.
• Update the readme:
  • Reorganise it to focus more on the package usage
  • Expand the installation instructions to cover some of the metric specific instructions
  • List metric references for import/CLI options