java: Start track with Bob

This is the complete skeleton for enabling Java, from curriculum
through the first exercise. Java is listed in the main curriculum.rb
but commented out; we will activate once there are a sufficient body
of exercises ported.

The java.png icon is taken from http://findicons.com/icon/88943/java?id=89126,
released under a freeware license.

Describe how to minimally set-up Java and JUnit. (Future work may
try to see if we can get Locale.additional_files working and provide
build integration via Gradle or Maven.)

The BobTest is a straight up port of the Python Bob.

This starts the path of fixing #933.
Merge pull request #946 from sit/java

java: Start track with Bob