Add script to build Unity package

Summary:
Added Bash script and bat file to build a package of Yoga in order to import Yoga for Unity easily.
Closes https://github.com/facebook/yoga/pull/288

Reviewed By: emilsjolander

Differential Revision: D4344198

Pulled By: splhack

fbshipit-source-id: 49ae27dbc513b4a2d161840ca5c5f4c9df2ab227

csharp/Unity/.gitignore

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+Yoga
+yoga.dll
+yoga.unitypackage

csharp/Unity/pack.sh

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+#!/bin/sh
+# Copyright (c) 2014-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree. An additional grant
+# of patent rights can be found in the PATENTS file in the same directory.
+
+cd "$( dirname "$0" )"
+
+if [ \! -f yoga.dll ]; then
+  echo "Launch win.bat on Windows and copy yoga.dll to here"
+  exit 1
+fi
+
+function build {
+  buck build $1
+  echo "$root/`buck targets --show-output $1|tail -1|awk '{print $2}'`"
+}
+
+function copy {
+  mkdir -p $3
+  cp $1 $3/$2
+}
+
+rm -rf Yoga yoga.unitypackage
+
+root=`buck root|tail -f`
+mac=$(build '//csharp:yoganet#default,shared')
+armv7=$(build '//csharp:yoganet#android-armv7,shared')
+ios=$(build '//csharp:yoganet-ios')
+win=yoga.dll
+
+Unity -quit -batchMode -createProject Yoga
+
+copy $win ${win##*/} Yoga/Assets/Facebook.Yoga/Plugins/x86_64
+copy $mac yoga Yoga/Assets/Facebook.Yoga/Plugins/x86_64/yoga.bundle/Contents/MacOS
+armv7path=Assets/Plugins/Android/libs/armeabi-v7a
+copy $armv7 ${armv7##*/} Yoga/$armv7path
+iospath=Assets/Plugins/iOS
+copy $ios ${ios##*/} Yoga/$iospath
+libs="$armv7path/${armv7##*/} $iospath/${ios##*/}"
+
+scripts=Yoga/Assets/Facebook.Yoga/Scripts/Facebook.Yoga
+mkdir -p $scripts
+(cd ../Facebook.Yoga; tar cf - *.cs)|tar -C $scripts -xf -
+
+tests=Yoga/Assets/Facebook.Yoga/Editor/Facebook.Yoga.Tests
+mkdir -p $tests
+(cd ../tests/Facebook.Yoga; tar cf - *.cs)|tar -C $tests -xf -
+
+Unity -quit -batchMode -projectPath `pwd`/Yoga -exportPackage Assets/Facebook.Yoga $libs `pwd`/yoga.unitypackage
+

csharp/Unity/win.bat

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+"C:\Program Files (x86)\MSBuild\14.0\Bin\MSBuild.exe" ..\Yoga\Yoga.vcxproj /p:configuration=Release /property:Platform=x64
+xcopy "..\Yoga\x64\Release\yoga.dll" %~dp0 /s /d /y