Added 'androidManifestFile' annotation processor option

To be able to support multiple flavors of an app in Android Studio with Gradle the manifest will almost always be unique per flavor. However the current version of the Transfuse annotation processor will try to find the AndroidManifest.xml automatically. But to be able to find it, the manifest file must be in the root folder. This means (but correct me if I'am wrong) that the manifest is shared across all flavors.

I wanted to use the options provided by the android-apt Gradle plugin to pass the location of the manifest file to the Transfuse annotation processor using the following snippet from the build.gradle file:
```
apt {
    arguments {
        androidManifestFile variant.processResources.manifestFile
    }
```
Is this a viable solution for Transfuse?

Credits for the AndroidAnnotations project which provides a similar solution.