Create a core-sources jar

*Created by: sampottinger*

This came up from @clankill3r but wanted to check with you @benfry before I did it. It would probably help folks using the core jar as a dependency in Java to have a core-sources jar. That's just adding:

```
<jar basedir="bin" destfile="library/core-sources.jar"> 
    <fileset dir="src" includes="**/*.java"/>
</jar>
```

Have any objections to that?