Add Gradle build files

These few files will allow developers using Android Studio to contribute!

Gradle also makes it super easy to bootstrap a build environment.

```
git clone https://github.com/siacs/Conversations.git
cd Conversations
git submodule update --init --recursive
./gradlew assembleDebug
```

You don't have to have gradle installed -- The `gradlew` script downloads the version of gradle prescribed in `gradle-wrapper.properties` to a local project cache in `./.gradle/`. This way all contributors are guaranteed to be using the same build tools!

Love your work. Cheers!