This adds the ability to specify a list of errors and warnings which
pep8 will ignore when running. This option is just passed directly
to the '--ignore' command line option.

Tools may now provide a 'check_dependencies' method which will be
called when initializing the worker, and refreshing tools. This method
should check the system for any dependencies the tool requires.

If dependencies are missing 'check_dependencies' should return false
and the worker will not listen on the tools queue, or send the tool in
it's list of installed tools.

This changes the Tool API slightly, and some tools may have to be
updated to work with the changes to the Tool class.

The PEP8 tool has been updated to check for the 'pep8' dependency.

Fix a few style issues and remove some unused code.

Ran pep8 tool against some code.
Removed the pep8 executable and tested the dependency checking.