Added feature to use rounded values

Summary:
Added an experimental feature to allow to use only rounded values. See #184. It's not a perfect solution and definitely  can be further improved. I'm looking forward to your ideas.
Closes https://github.com/facebook/css-layout/pull/256

Reviewed By: splhack

Differential Revision: D4214168

Pulled By: emilsjolander

fbshipit-source-id: 6293352d479b7b4dad258eb3f9e0afaa11cf7236

enums.py

@@ -77,7 +77,9 @@ ENUMS = {
         'Debug',
         'Verbose',
     ],
-    'CSSExperimentalFeature': [],
+    'CSSExperimentalFeature': [
+        'Rounding',
+    ],
     'CSSPrintOptions': [
         ('Layout', 1),
         ('Style', 2),