Add letterSpacing style property for Text

Summary:
Fixes #457
Closes https://github.com/facebook/react-native/pull/482
Github Author: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Examples/UIExplorer/TextExample.ios.js

@@ -218,6 +218,26 @@ exports.examples = [
       </View>
     );
   },
+}, {
+  title: 'Letter Spacing',
+  render: function() {
+    return (
+      <View>
+        <Text style={{letterSpacing: 0}}>
+          letterSpacing = 0
+        </Text>
+        <Text style={{letterSpacing: 2, marginTop: 5}}>
+          letterSpacing = 2
+        </Text>
+        <Text style={{letterSpacing: 9, marginTop: 5}}>
+          letterSpacing = 9
+        </Text>
+        <Text style={{letterSpacing: -1, marginTop: 5}}>
+          letterSpacing = -1
+        </Text>
+      </View>
+    );
+  },
 }, {
   title: 'Spaces',
   render: function() {

Examples/UIExplorer/UIExplorerTests/UIExplorerTests.m

@@ -88,35 +88,19 @@
   XCTAssertTrue(foundElement, @"Cound't find element with '<View>' text in %d seconds", TIMEOUT_SECONDS);
 }
 
-- (void)testViewExampleSnapshot
-{
-  [_runner runTest:_cmd module:@"ViewExample"];
+#define RCT_SNAPSHOT_TEST(name, reRecord) \
+- (void)test##name##Snapshot              \
+{                                         \
+  _runner.recordMode |= reRecord;         \
+  [_runner runTest:_cmd module:@#name];   \
 }
 
-- (void)testLayoutExampleSnapshot
-{
-  [_runner runTest:_cmd module:@"LayoutExample"];
-}
-
-- (void)testTextExampleSnapshot
-{
-  [_runner runTest:_cmd module:@"TextExample"];
-}
-
-- (void)testSwitchExampleSnapshot
-{
-  [_runner runTest:_cmd module:@"SwitchExample"];
-}
-
-- (void)testSliderExampleSnapshot
-{
-  [_runner runTest:_cmd module:@"SliderExample"];
-}
-
-- (void)testTabBarExampleSnapshot
-{
-  [_runner runTest:_cmd module:@"TabBarExample"];
-}
+RCT_SNAPSHOT_TEST(ViewExample, NO)
+RCT_SNAPSHOT_TEST(LayoutExample, NO)
+RCT_SNAPSHOT_TEST(TextExample, NO)
+RCT_SNAPSHOT_TEST(SwitchExample, NO)
+RCT_SNAPSHOT_TEST(SliderExample, NO)
+RCT_SNAPSHOT_TEST(TabBarExample, NO)
 
 // Make sure this test runs last
 - (void)testZZZ_NotInRecordMode