Android MessageNano Protocol Buffer Converter class

We are evaluating the use of Retrofit on an Android app project within YouTube. This library is amazing. :) We use `com.google.protobuf.nano.MessageNano` based protos, which are lighter weight, have far fewer methods, use arrays instead of lists, and just create generally smaller code for Android projects. It also doesn't require reflection on the parse method, making it safer for Proguard.

We've been testing this implementation locally, and it'd be great to upstream it for others to use, as well as to avoid having a one-off `Converter` in our codebase.

I separated the build module for this into a separate `nano` Maven profile due to the requirement of Android's external_protobuf. Running `mvn clean verify -P nano` works fine with their protobuf implementation installed, and `mvn clean verify` alone ignores the `protobuf-nano` module, 

`checkstyle_result.xml` did not report any style issues.

The unit test is basically identical to the regular `ProtoConverter` test, with the API updated to be nano-compatible.