feat(locators): add toString() wrapper for this.message

Adding `toString()` on **webdriver.Locator** to be consistent with [webdriver's locators API][1]

This is useful when writing custom matchers.

Other way around would be extending [webdriver's locators API][1] with the `message` property.

In case you want to merge this idea it might be wise to avoid code repetition by creating a base object to extend from.

[1]: https://code.google.com/p/selenium/source/browse/javascript/webdriver/locators.js#249