Structured approach to invoking command line

[rayh]

Such that:

• Xcode is isolated from the idiosyncrasies of command line option escaping, etc
• Third-parties can intercept and modify built in commands
• Third-parties can create new commands within a particular build context (builder.rb)

Perhaps by introduction of a Xcode::Command class that encapsulates the command and its arguments that is then sent to the Xcode::Shell.execute() method.

[rayh]

We now have a Xcode::Shell::Command object which represents a structured command line, but we dont have a standard approach to creating/mutating the object.