backicon.png: error: Invalid filename. Unable to add.

There seems to be an issue with the naming convention used for the back-icon.png. I got an error trying to assemble the Android app. iOS works fine.

When running `./gradlew aseembleRelease` for Android, the build fails specifically with:

```
Unknown source file : /Users/joelcloralt/code/MyApp/android/app/build/intermediates/res/merged/release/drawable-mdpi/node_modules_reactnativerouterflux_node_modules_reactnativeexperimentalnavigation_assets_backicon.png: error: Invalid filename. Unable to add.
```

I'm using:

```
"react": "^15.1.0",
"react-native": "^0.27.2",
"react-native-router-flux": "3.30.0",
```

I _solved_ the issue by changing [this line](https://github.com/aksonov/react-native-experimental-navigation/blob/master/NavigationHeaderBackButton.js#L37) in `NavigationHeaderBackButton.js` to simply use `require('./back_chevron.png')`. Then deleted the `app/build` folder and ran the aseembleRelase again and that worked. This is just a workaround so that's why I'm submitting the ticket.

Here is a little bit more from the build log.

```
bundle: start
bundle: finish
bundle: Writing bundle output to: /Users/joelcloralt/code/MyApp/android/app/build/intermediates/assets/release/index.android.bundle
bundle: Done writing bundle output
bundle: Copying 22 asset files
bundle: Done copying assets
:app:processReleaseManifest UP-TO-DATE
:app:processReleaseResources
Unknown source file : /Users/joelcloralt/code/MyApp/android/app/build/intermediates/res/merged/release/drawable-mdpi/node_modules_reactnativerouterflux_node_modules_reactnativeexperimentalnavigation_assets_backicon.png: error: Invalid filename. Unable to add.

:app:processReleaseResources FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:processReleaseResources'.
> com.android.ide.common.process.ProcessException: org.gradle.process.internal.ExecException: Process 'command '/Users/joelcloralt/Library/Android/sdk/build-tools/23.0.1/aapt'' finished with non-zero exit value 1

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

```