Add source files to jar

[Tarrasch]

This makes is easier for people importing the jar to drill down through
definitions in the Java IDE of their choice.

This will also present javadocs in your IDE when available.

[Tarrasch]

I forgot to mention. I was highly inspired by this StackOverflow when writing this: http://stackoverflow.com/questions/5063630/generate-jars-with-source-code-using-ant

[mimno]

How much does this affect the file size? There are some cases (the R interface, for example) where it's good to keep the jar small. That might be the exception, however.

[Tarrasch]

Using du --human-readable dist/mallet.jar, I get from 2.2M (before) to 5,7M (after)

[Tarrasch]

@mimno, is there any other information I need/should provide?