Added an initial default.nix that can build webdriver-w3c.

[seanparsons]

This is a very early cut of a file that'll get webdriver-w3c to be built via nix, so it can be built with just nix-build . in the root of the project and it'll do all the heavy lifting.

The only particularly unusual thing is the dontCheck wrapper around script-monad and webdriver-w3c which just prevents their test suites from being built and run. For script-monad this is because it attempts to make a network call which nix prevents so that builds can't break the reproducibility of the build. Similarly the webdriver-w3c test suite relies on instances of geckodriver and chromedriver. Both of those projects could do with a build flag that disable any tests which rely on those and then eventually get these packages flagged as good in nixpkgs with that flag set.

[nbloomf]

Thank you for making this PR! I don't know much about Nix and this will be a good excuse to learn.

Leaving a note for my future self: look into splitting up the test suites to isolate the ones that make nix cranky.