[change] new accessibility features and docs

* Change 'accessible' to align with React Native.
* Add support for 'importantForAccessibility'.
* Stop event propagation for keyboard-activated Touchables (nested
  Touchables now respond the same as when touch-activated).
* Fix whitespace layout of nested Text elements.
* Use 'div' for Text to improve TalkBack grouping.
* Rewrite accessibility docs.

Close #382
Fix #408

docs/components/Button.md

@@ -6,23 +6,27 @@ build your own custom button using `TouchableOpacity` or
 
 ## Props
 
-**accessibilityLabel**: string
+**accessibilityLabel**: ?string
 
-Defines the text available to assistive technologies upon interaction with the
-element. (This is implemented using `aria-label`.)
+Overrides the text that's read by a screen reader when the user interacts
+with the element.
 
-**color**: string
+**color**: ?string
 
 Background color of the button.
 
-**disabled**: bool = false
+**disabled**: ?boolean
 
-If true, disable all interactions for this component
+If `true`, disable all interactions for this element.
 
 **onPress**: function
 
 This function is called on press.
 
+testID: ?string
+
+Used to locate this view in end-to-end tests.
+
 **title**: string
 
 Text to display inside the button.