BREAKING: Create ShellScript subclass

[jmgate]

Create a ShellScript subclass of StagedScript and move all the run() and commands_executed functionality, etc., into the subclass. This way, users who have no desire to interact with the underlying shell have no need to worry about any of that. It also sets us up for a future in which we develop a more powerful Shell class to augment the functionality provided by subprocess (along the lines of shell-logger).

Note: This will be a breaking change, as existing users will need to inherit from a different class.