Changed argument array handling to correct errors.

After struggling to get my avows.json to run properly, I realized that there was an issue parsing out the arguments when calling the shell. I was attempting to perform as task such as "chmod -x build.sh" and it ended up with "chmod -x,build.sh" which didn't really work as you would imagine. At first, I took off the square brackets around args in the default case of the switch statement, and that fixed the issue I was having, but I decided to go ahead and fix the other sections of the page to prevent any possible future problems. It seems as though if child_process.spawn gets an array inside of an array, instead of merging them, it does something like args.join(','), which is what I could gather from the log file. Anyways, accepting the pull request should allow for multi-argument shell function calls to work properly. I can understand why this got missed, since npm and grunt are both used in the example with only one argument. Great project, you really made my day when I found this! Keep up the good work!