Add Java sample project

[drekbour]

Now Mixite is written in Kotlin (still a new kid on the block), it suddenly becomes a little scary for Java devs (despite saying Don't Panic on the cover). It might do well to provide a basic Java usage of mixite.core both

• Candy for Java-only eyes (can be linked to from README.md)
• Regression testing the Java-facing API (c.f. Accessing boolean fields in Kotlin using is prefix: isSomething() #42)

[adam-arold]

This is a good idea! We have a java skeleton for Zircon so it would make sense to have it for Mixite as well.