Deprecate `getBrowsers` and replace it with `getAllBrowsers`, `getSeleniumBrowsers` and `getWebDriverBrowsers`

The official documentation (https://saucelabs.com/docs/rest) no longer uses http://saucelabs.com/rest/v1/info/browsers, but http://saucelabs.com/rest/v1/info/browsers/all, http://saucelabs.com/rest/v1/info/browsers/selenium-rc and http://saucelabs.com/rest/v1/info/browsers/webdriver to retrieve the list of browsers.

The biggest change I've noticed is that the old URL used the property `selenium_name`, while the new ones use `api_name`.

I haven't removed the `getBrowsers` method to keep backwards compatibility, but I've removed references to it in the *README* to discourage use. I've also added `getAllBrowsers`, `getSeleniumBrowsers` and `getWebDriverBrowsers`.

What do you think?