use java syntax highlighting in README.md

With the amount of sample code in the documentation the readability could be improved by syntax highlighting via Github flavored markdown (GFM)

It even works with the Automatic Page Generator the project uses

See:
http://xorrr.github.io/de.flapdoodle.embed.mongo/

The next step now would be to remove the indentation of the old formatting in the code blocks. I guess i'd leave that to you so you don't have to doublecheck if i maliciously screwed with your code samples.