feat(runner): allow bypassing the selenium standalone server if running only chrome

Using the config option `chromeOnly` now enables running ChromeDriver directly,
without going through the Selenium Standalone. The chromedriver binary should be
available in your PATH, or should be specified with the congif option
`chromeDriver`.
feat(runner): allow bypassing the selenium standalone server if running only chr

Using the config option `chromeOnly` now enables running ChromeDriver directly,
without going through the Selenium Standalone. The chromedriver binary should be
available in your PATH, or should be specified with the config option
`chromeDriver`.
feat(element): element.all now has 'first' and 'last' methods

Return the first or last element that matches a locator. Closes #171.