Some tools require Java to be set up and for .jar files to be made

available. These tend to be more complicated than typical tools. We only

have one in the codebase right now, Checkstyle, which was written to

have its own special configuration key for specifying the .jar file to

use, but it was a bit hacky and painful to manage.
This change introduces new configuration and a mixin for Java-based

tools.
A new java_classpath setting in the worker configuration can be set to

a list of .jar files or directories containing .jar files. These

will all be added to the class path when running Java programs.
A new JavaToolMixin handles all the Java runtime management for a

tool. Subclasses set the name of the class that will run the program.

The mixin takes care of building a base command line and a class path

(based on the configuration and on the $CLASSPATH environment

variable). It also handles dependency checks, making sure that java

and the class defined on the tool can both be run.
This will soon be used by CheckstyleTool.

All unit tests passed on Python 2.7 and 3.x.
Used this with the upcoming CheckstyleTool change.