Summary:
Adds the feature to use percentage as a value unit.
You can use the function ```YGPx(float)``` and ```YGPercent(float)``` for convenience.
I did some benchmarks:
```
Without Percentage Feature - Release x86:
Stack with flex: median: 0.000000 ms, stddev: 0.146683 ms
Align stretch in undefined axis: median: 0.000000 ms, stddev: 0.136525 ms
Nested flex: median: 0.000000 ms, stddev: 0.490101 ms
Huge nested layout: median: 23.000000 ms, stddev: 0.928291 ms
Stack with flex: median: 0.000000 ms, stddev: 0.170587 ms
Align stretch in undefined axis: median: 0.000000 ms, stddev: 0.143384 ms
Nested flex: median: 0.000000 ms, stddev: 0.477791 ms
Huge nested layout: median: 22.000000 ms, stddev: 2.129779 ms
With Percentage Feature - Release x86:
Stack with flex: median: 0.000000 ms, stddev: 0.132951 ms
Align stretch in undefined axis: median: 0.000000 ms, stddev: 0.136525 ms
Nested flex: median: 0.000000 ms, stddev: 0.489570 ms
Huge nested layout: median: 21.000000 ms, stddev: 1.390476 ms
Closes https://github.com/facebook/yoga/pull/258
Reviewed By: dshahidehpour
Differential Revision: D4361945
Pulled By: emilsjolander
fbshipit-source-id: a8f5bc63ad352eb9410d792729e56664468cd76a
