Add justify-content: space-evenly

Summary:
Adds new ```space-evenly``` for ```justify-content```.

Also adds a typofix in one of the other justify-content tests.

Fixes #657
Closes https://github.com/facebook/yoga/pull/658

Differential Revision: D6407996

Pulled By: emilsjolander

fbshipit-source-id: cc837409e1345624b4bd72c31e25fe68dcb0f6a3

yoga/Yoga.cpp

@@ -2447,6 +2447,11 @@ static void YGNodelayoutImpl(const YGNodeRef node,
             betweenMainDim = 0;
           }
           break;
+        case YGJustifySpaceEvenly:
+          // Space is distributed evenly across all elements
+          betweenMainDim = remainingFreeSpace / (itemsOnLine + 1);
+          leadingMainDim = betweenMainDim;
+          break;
         case YGJustifySpaceAround:
           // Space on the edges is half of the space between elements
           betweenMainDim = remainingFreeSpace / itemsOnLine;