Add signing config to gradle

This way I (and anyone) can align and sign the release builds by just
excecuting ```./gradlew assembleRelease```, which will save me time
and allow me to upload an signed apk with each version.

Author: raatmarien

.gitignore

@@ -1,3 +1,10 @@
+*~
+*#
+keystore.properties
+app/keystore.ks
+app/build.gradle
+
+
 # Created by https://www.gitignore.io
 
 ### Android ###

app/build.gradle

@@ -5,6 +5,31 @@ ext {
 apply plugin: 'com.android.application'
 apply plugin: 'kotlin-android'
 
+
+if(rootProject.file("keystore.properties").exists()) {
+    // From https://developer.android.com/studio/publish/app-signing.html
+    // to load the signing config file
+    def keystorePropertiesFile = rootProject.file("keystore.properties")
+    def keystoreProperties = new Properties()
+    keystoreProperties.load(new FileInputStream(keystorePropertiesFile))
+
+    android {
+        signingConfigs {
+            release {
+                keyAlias keystoreProperties['keyAlias']
+                keyPassword keystoreProperties['keyPassword']
+                storeFile file(keystoreProperties['storeFile'])
+                storePassword keystoreProperties['storePassword']
+            }
+        }
+        buildTypes {
+            release {
+                signingConfig signingConfigs.release
+            }
+        }
+   }
+}
+
 android {
     compileSdkVersion 25
     buildToolsVersion "25.0.2"