Issue 553 - Sauce Labs and BrowserStack verbiage tightening (cypress-io#767)

* fix typo/rewrite Sauce Labs section to be explicit

Remove article 'a' from Capybara section 
Move statements about Sauce Labs integration to the bottom, wrap in info box, and use explicit language that integration is not currently available.

* add Sauce Labs/BrowserStack question/answer

Add question, Is Cypress compatible with Sauce Labs and BrowserStack? 
Link to general-questions-faq and issue 310

* edit Sauce Labs/BrowserStack answer

reword answer to be explicit about Cypress's compatibility with Webdriver. Remove ambiguous remark about "discussions"

* remove mention of 'Selenium' from Sauce Labs

* fix typo-grammar/spelling

Cypress's --> Cypress'
Webdriver ---> WebDriver

* fix typo 

Cypress's --> Cypress'

* tweak content for sauce labs + browserstack explanation

* cleanup verbaige around sauce/browser stack

Author: RandallKent

source/faq/questions/using-cypress-faq.md

@@ -297,7 +297,7 @@ You can read more about parallelization {% issue 64 'here' %}.
 
 ## {% fa fa-angle-right %} Is Cypress compatible with Sauce Labs and BrowserStack? 
 
-Cypress’ API is compatible with WebDriver specific tasks that Sauce Labs and BrowserStack use to launch browsers. Because Cypress currently only supports Chrome* based browsers, we have not yet added integration for these services. When cross browsers are added, Cypress will add full integration with Sauce Labs and BrowserStack.
+Our goal is to offer full integration with Sauce Labs and BrowserStack in the future, however, complete integration is not yet available.
 
 ## {% fa fa-angle-right %} Can I run a single test or group of tests?