This updates CheckstyleTool to subclass the newer BaseTool, and to

use the newer features it offers (dependencies, improved file extension

handling, centralized base command building, new comment options, and

Java tool mixin).
Configuration for checkstyle has improved. It previously required a full

XML file body to be set, which was a lot to manage. Now, to simplify

things, we also allow a Checkstyle-bundled XML file to be specified

(google_checks.xml or sun_checks.xml). If provided, we'll use that

instead of assuming the content is an XML document body.
They could also potentially drop a .jar file in the class path with

XML files, and reference that in the Configuration XML option.
Parsing has improved, handling syntax errors with configurations or

source code and leaving a suitable comment. For other errors, we now

include column, severity, and error code information in the comment.
Python 3 compatibility was also fixed. make_tempfile() was being

called with a Unicode string for the configuration contents, but

requires a byte string.

Unit tests pass on Python 2.7 and 3.x.