Fix #63: Use auto-value for Event classes.

@hamidp I'm stealing this from you ;-o

What does it mean? When there is only a dependencies section like following:
``
dependencies {
    provided "com.google.auto.value:auto-value:1.0-rc1+"
}
```

I got this error:

```
:sample-app:packageDebug
Error: duplicate files during packaging of APK /Users/morrita/w/RxAndroid/sample-app/build/outputs/apk/sample-app-debug-unaligned.apk
        Path in archive: META-INF/services/javax.annotation.processing.Processor
        Origin 1: /Users/morrita/.gradle/caches/modules-2/files-2.1/com.google.auto.service/auto-service/1.0-rc1/f4f5ad098bf9a3c4fc95c15214c4ea014c38ac39/auto-service-1.0-rc1.jar
        Origin 2: /Users/morrita/.gradle/caches/modules-2/files-2.1/com.google.auto.value/auto-value/1.0-rc1/95f1e38ae958b9b380846886913b1974065be454/auto-value-1.0-rc1.jar
You can ignore those files in your build.gradle:
        android {
          packagingOptions {
            exclude 'META-INF/services/javax.annotation.processing.Processor'
          }
        }
:sample-app:packageDebug FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':sample-app:packageDebug'.
> Duplicate files copied in APK META-INF/services/javax.annotation.processing.Processor
        File 1: /Users/morrita/.gradle/caches/modules-2/files-2.1/com.google.auto.service/auto-service/1.0-rc1/f4f5ad098bf9a3c4fc95c15214c4ea014c38ac39/auto-service-1.0-rc1.jar
        File 2: /Users/morrita/.gradle/caches/modules-2/files-2.1/com.google.auto.service/auto-service/1.0-rc1/f4f5ad098bf9a3c4fc95c15214c4ea014c38ac39/auto-service-1.0-rc1.jar
```

What's wrong?