Add percentage support to react native

Summary:
Adds support for percentage value in react native.

syntax: property: 100 | property | '100%'

supported properties:
padding
margin
width
height
minWidth
minHeight
maxWidth
maxHeight
flexBasis

```
class Playground extends React.Component {
  render() {
    return (
      <View style={{backgroundColor: 'white', padding: 10, paddingTop: 30, height: '100%'}}>
        <Text>
          If you want to quickly test out something,
          open the Playground.js file and start coding.
        </Text>
        <View style={{backgroundColor: 'red', height: 50, width: 50}}/>
        <View style={{backgroundColor: 'blue', height: '50%', width: '50%'}}/>
      </View>
    );
  }
}
```

Reviewed By: astreet

Differential Revision: D4376549

fbshipit-source-id: c41d68a7555396f95d063a7527ee081773ac56dc

Examples/UIExplorer/UIExplorerUnitTests/RCTConvert_YGValueTests.m

@@ -0,0 +1,46 @@
+/**
+ * The examples provided by Facebook are for non-commercial testing and
+ * evaluation purposes only.
+ *
+ * Facebook reserves all rights not expressly granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON INFRINGEMENT. IN NO EVENT SHALL
+ * FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#import <XCTest/XCTest.h>
+
+#import <React/RCTConvert.h>
+#import <React/RCTUtils.h>
+
+@interface RCTConvert_YGValueTests : XCTestCase
+
+@end
+
+@implementation RCTConvert_YGValueTests
+
+- (void)testUndefined
+{
+  YGValue value = [RCTConvert YGValue:nil];
+  XCTAssertEqual(value.unit, YGUnitUndefined);
+}
+
+- (void)testNumberPoints
+{
+  YGValue value = [RCTConvert YGValue:@100];
+  XCTAssertEqual(value.unit, YGUnitPixel);
+  XCTAssertEqual(value.value, 100);
+}
+
+- (void)testStringPercent
+{
+  YGValue value = [RCTConvert YGValue:@"100%"];
+  XCTAssertEqual(value.unit, YGUnitPercent);
+  XCTAssertEqual(value.value, 100);
+}
+
+@end