Create a class or module for interacting with `ps`

[jmoody]

In many places, we call ps in backticks and then send strip (or some other reader) to the resulting string. This is dangerous for us because the string might contain UTF-8 characters which will result in a crash.

The CoreSimulator class has an example of processing ps output using RunLoop::Shell.

Related to Pass all read calls through utf-8 filter #230