Overall accessibility improvements

Changed/added accessibility labels on:

* Arrivals and departures table (now reads, "Route toward Destination, expected minutes until arrival/since departure at time, trip status") 
* Removed VO selectivity of table headings (causes more swipes to get to information, and context is now embedded in label)
* Changed "list" to "nearby stops list" on map view (suggestion from blind users)
* Added labels to SVWebViewController (used to read "SVWebViewControllerBackButton" etc. so I changed it to "Back, web view button") and differentiated between the back button that returns the user to the stop arrival times.

I made these adjustments after consulting with a blind researcher who is currently working with me, and who has been using OBA iPhone for awhile. I also followed accessibility research which indicates that labels should use less wording and read salient information first. We'd like to test out how the arrival table label works with new blind users, which we will have an opportunity to do in our StopFinder study. 

Thoughts on any of these changes? I'm not completely set on the wording, but as stated previously, I'd like it to be concise and informative. For reference, accessibility labels should describe the UI element, read any text associated with the label, and/or indicate the action it takes. Accessibility hints are generally for new users who could benefit from additional information about the element; these are often turned off by more advanced VO users, according to my collaborator.