Add partially delivered message status

Initially driven by the work to add more supported statuses to the supported message status list.
Included in this feature change is the move to a Maven build process.

After cloning the repo, the project can be built with Java 7 and Maven as follows:
```bash
mvn package
```

The project can be opened with JetBrains IntelliJ IDEA using the "Import Project" method and selecting "Maven" in the list of external models. The default settings are fine.