adding system property for databases as part of #36

The Hibernate config file is now settable via the system property `mamute.database`. The value defaults to the original '/hibernate.cfg.xml' value if the property is not specified.

Also added a basic configuration for running the tests with H2. The Maven build will default to using the MySQL configuration, but you can override it like so:

```
mvn clean package -Dmamute.database=h2
```