Support other whitespace in WordSplitter

Our Carbon setup at work, some of which is custom-built, typically uses
tabs and not spaces in the Carbon plaintext protocol. The stock Carbon
daemons are fine with this, as are some related tools like
graph-explorer.

However, Kairos's carbon server only accepts spaces, and throws
exceptions for every received line using tabs. This change makes it
split strings on any (iso-8859-1) whitespace characters without
sacrificing much of the speed improvement over String.split() or
Pattern.split(). It also allows leading or trailing whitespace on input
lines.

As far as I can tell, this does not break any other uses of
the WordSplitter class.