[Text] Support "textAlign: justify" in Text components

Summary:
Finished adding support for justified text on iOS... just had to expose a prop value.
Fixes #529

Closes #1486
Github Author: James Ide <ide@jameside.com>

Test Plan:
 View the Text example in the UIExplorer and see that all but the last lines of text in the justified text example are justified and extend to the far right of the available space. Tested on iOS 8, iPhone 6 simulator.

![text](https://cloud.githubusercontent.com/assets/379606/7926252/a90d2e98-0884-11e5-9f12-5cbb27505a0b.png)

Examples/UIExplorer/TextExample.ios.js

@@ -215,6 +215,11 @@ exports.examples = [
         <Text style={{textAlign: 'right'}}>
           right right right right right right right right right right right right right
         </Text>
+        <Text style={{textAlign: 'justify'}}>
+          justify: this text component's contents are laid out with "textAlign: justify"
+          and as you can see all of the lines except the last one span the
+          available width of the parent container.
+        </Text>
       </View>
     );
   },

Examples/UIExplorer/UIExplorerIntegrationTests/UIExplorerIntegrationTests.m

@@ -42,7 +42,7 @@ - (void)setUp
   _runner = RCTInitRunnerForApp(@"Examples/UIExplorer/UIExplorerApp.ios");
 
   // If tests have changes, set recordMode = YES below and run the affected
-  // tests on an iPhone5, iOS 8.1 simulator.
+  // tests on an iPhone5, iOS 8.3 simulator.
   _runner.recordMode = NO;
 }
 

Libraries/Text/TextStylePropTypes.js

@@ -26,8 +26,9 @@ var TextStylePropTypes = Object.assign(Object.create(ViewStylePropTypes), {
   lineHeight: ReactPropTypes.number,
   color: ReactPropTypes.string,
   containerBackgroundColor: ReactPropTypes.string,
+  // NOTE: "justify" is supported only on iOS
   textAlign: ReactPropTypes.oneOf(
-    ['auto' /*default*/, 'left', 'right', 'center']
+    ['auto' /*default*/, 'left', 'right', 'center', 'justify']
   ),
   writingDirection: ReactPropTypes.oneOf(
     ['auto' /*default*/, 'ltr', 'rtl']