always use full paths when starting scripts

In response to [this issue](https://github.com/nodejitsu/forever/issues/364).

There are two downsides I can see to this.

One is that the command-line output of `forever list` will now be a little bulkier.

The other is that some people might currently rely on the ability to stop identically-named scripts with a single command. However, that would mean they're relying on undocumented functionality that could be classified as a bug anyway.