create an app keystore and sign it

.gitignore

@@ -1,7 +1,9 @@
 *.iml
 .gradle
 /local.properties
+/gradle.properties
 /.idea/
 .DS_Store
 /build
 /captures
+release.keystore

app/build.gradle

@@ -1,9 +1,16 @@
 apply plugin: 'com.android.application'
 
 android {
+    signingConfigs {
+        config {
+            keyAlias RELEASE_KEY_ALIAS
+            keyPassword RELEASE_KEY_PASSWORD
+            storeFile file('../release.keystore')
+            storePassword RELEASE_STORE_PASSWORD
+        }
+    }
     compileSdkVersion 23
     buildToolsVersion "22.0.1"
-
     defaultConfig {
         applicationId "calculator.simplemobiletools.com.simple_calculator"
         minSdkVersion 16
@@ -15,8 +22,9 @@ android {
     }
     buildTypes {
         release {
-            minifyEnabled false
+            minifyEnabled true
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
+            signingConfig signingConfigs.config
         }
     }
 }
@@ -26,10 +34,8 @@ dependencies {
     compile 'com.android.support:appcompat-v7:23.1.1'
     compile 'com.jakewharton:butterknife:7.0.1'
     compile 'me.grantland:autofittextview:0.2.1'
-
     testCompile 'junit:junit:4.12'
     testCompile 'org.robolectric:robolectric:3.0'
-
     androidTestCompile 'com.android.support:support-annotations:23.1.1'
     androidTestCompile 'com.android.support.test:runner:0.4.1'
     androidTestCompile 'com.android.support.test:rules:0.4.1'