Allow the fork to Marshal a result back over a pipe

[nevinera]

Right now we have a pipe open between the parent and child so that any ruby exceptions encountered can be marshaled back across it and re-raised. It would be equally useful to be able to conveniently return a result from the block.

Add an option to 'retrieve_result' (it'll default to false), which will cause a result-pipe to be opened, and the child process to marshal and send the result of the block back to the parent. The result will be available instead of the child's status object, and will be nil if the child does not complete successfully and without exiting.