Add scripts for performing common tasks to make it easier to contribute

It's easy to contribute when everything is right there, sitting in front of you, with semantic names. Want to boot up the server? Run `script/server`. Want to install the dependencies of the project? No problem. Just run `script/bootstrap`.

These kinds of scripts make it easier to contribute to a site, both for external contributors (like me!) and for internal contributors, like you.
Not sure why making an empty `_site` dir would help -- there'd be nothing to serve, and `jekyll serve` does a build process that would make `_site` anyway, right?