Java OutputStream 64kiB limit fix

Issue #40 is caused because [Java's `OutputStream`s can only handle a write of up to 64kiB at a time](http://www.drillio.com/en/software-development/java/encoded-string-too-long-64kb-limit/). Spark currently writes the entire web page at once, so if the page is larger than 64kiB, it gets truncated.

This fixes it by changing `spark.webserver.MatcherFilter` to write a 64kiB block at a time.