Update build.gradle (#27)

Author: maxmamis

android/build.gradle

@@ -1,13 +1,21 @@
 
 apply plugin: 'com.android.library'
 
+def _ext = rootProject.ext
+
+def _compileSdkVersion = _ext.has('compileSdkVersion') ? _ext.compileSdkVersion : 23
+def _buildToolsVersion = _ext.has('buildToolsVersion') ? _ext.buildToolsVersion : '23.0.1'
+def _minSdkVersion = _ext.has('minSdkVersion') ? _ext.minSdkVersion : 16
+def _targetSdkVersion = _ext.has('targetSdkVersion') ? _ext.targetSdkVersion : 22
+
+
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+    compileSdkVersion _compileSdkVersion
+    buildToolsVersion _buildToolsVersion
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion _minSdkVersion
+        targetSdkVersion _targetSdkVersion
         versionCode 1
         versionName "1.0"
         ndk {