Add selenium-chromedriver

Automate process of downloading and installing ChromeDriver binary. This
simplifies the development workflow: the only prerequisites are Google
Chrome and npm.
Add TravisCI configuration file

The Chrome installation script comes from the discussion on the TravisCI
issue tracker, "Chromium/Chrome does not work in an OpenVZ container"
(gh-938):

https://github.com/travis-ci/travis-ci/issues/938
Modify TravisCI install script
Add tests
Add test for updating item
Add tests for task completion
Add test for persistence
Move selectors to declarative JSON file
Automate selector object creation
Create (and use) an "application driver" class