Add default help when no parameter is provided. Added -c parameter to specify cloudfront id.

[phzbox]

Hi, I had to fix this for myself so I thought I might as well pull request it to you.

1. A bit confusing when typing s3front without any parameters. It throws an exception because -k is missing. I prefer to show by default the help message in that case.
2. For me, it couldn't match the right cloudfront distribution to invalidate. I prefer passing the distribution ID, so I added a -c || --cloudfront . Also, there was no error message if no distribution were found so I added an error message and process.exit(). Otherwise it was just hanging.