Add dependency file

[emazzotta]

It's quite common to include a requirements.txt file in a python project which specifies all the dependencies so that those can be installed in one command as described in the readme.

This change will:

• Add a requirements.txt file
• Update the readme on how to install the specified dependencies
• Make the demo executable (thanks to shebang + chmod on the file)

Please note that before, as well as after my changes, I have the same issue as described here: https://github.com/HackerPoet/PySpace/issues/1 so I'm not able to verify the full functionality.

[HackerPoet]

Can those requirements be made >= instead of ==? I'm not sure what the absolute minimum versions are, but I'd like it to be future-proof.

[emazzotta]

Sure, that's a fair point. I just amended my change.

[HackerPoet]

Thanks!