Conversion to Gradle and Fixed #55

* Fixed #55 
* Converted to gradle,
   * for Play Store releases: create folder called ".signing" in $HOME, copy the signing keystore to that folder and call it "metalonly.keystore", place the code below in a file called "metalonly.gradle", replace the password with the correct one
   * that way building and signing via gradle is possible, without putting the password under version control and without entering them on every release

```
android {
  signingConfigs {
    release {
      storeFile file(project.property("metalonly.signing")+".keystore")
      storePassword "STORE_PASSWORD"
      keyAlias "ALIAS"
      keyPassword "KEY_PASSWORD"
    }
  }

  buildTypes {
    release {
      signingConfig signingConfigs.release
    }
  }
}´´´