To easily adhere to LGPL, prevent obfuscation

As the author of this library suggested in #115, it is recommended to avoid obfuscation for libraries that use LGPL license as it could be considered as modification. This requires adding proguard rules. Either the user can add the rules or the library itself can provide these rules.

It would be better if this library itself provides a consumer proguard file that prevents this obfuscation. This makes it easy as most users may forget to add these rules manually otherwise. More details about gradle's `consumerProguardFiles` can be found [here](https://developer.android.com/studio/projects/android-library#Considerations).