First step to sanity testing

This represents the first step towards release testing, issue #992:

* [ ] use wget to test for dead links (can't be done until we have no dead links)
* [x] use webdriver to test a few dartlang.org features
* [x] document in README

With this change, one can test:

* one's local copy of dartlang.org with:
  * `bundle exec ruby src/tests/site/sanity.rb`
  * or `make sanity-test` for convenience
* the live dartlang.org with:
  * `bundle exec ruby src/tests/site/sanity.rb https://www.dartlang.org/`
  * or `make sanity-test-live` for convenience