Update build.gradle to allow rootProject overrides of versions

Author: cbrevik

android/build.gradle

@@ -1,18 +1,22 @@
 apply plugin: 'com.android.library'
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+    compileSdkVersion safeExtGet('compileSdkVersion', 23)
+    buildToolsVersion safeExtGet('buildToolsVersion', "23.0.1") 
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 22)
         versionCode 1
         versionName "0.0.1"
     }
 }
 
 dependencies {
-    compile 'com.facebook.react:react-native:+'
-    compile 'com.anjlab.android.iab.v3:library:1.0.44'
+    compile "com.facebook.react:react-native:${safeExtGet('reactNativeVersion', '+')}"
+    compile "com.anjlab.android.iab.v3:library:${safeExtGet('anjlabIABVersion', '1.0.44')}"
 }