Add a mechanism to return a string containing the command to be run

[jvlcek]

A mechanism is needed that would provide the caller with a string representing the command to be run. It could be returned from run and run! also returned from a new "what would be run if" method. Where a hash could be passed in, much the way it is to run!, and a string would be returned containing the command that would be run.

[Fryguy]

I exposed #build_command_line in the awesome_spawn gem, which will be available once #86 is merged.

[kbrock]

This has been merged (close?)

[carbonin]

@bdunne This can be closed right?