Make existsSync work with older versions of Node

On older versions of Node, attempts to use fs.existsSync results in a crash with error:
`TypeError: Object #<Object> has no method 'existsSync'`

Unfortunately at this time, the [maximum version of Node available on appFog](https://docs.appfog.com/languages/node) is 0.8.14, which exhibits this error.

This change is a workaround for that issue.  It does a basic null check for fs.existsSync.  If it doesn't exist, it requires the 'path' module and uses path.existsSync instead.  If fs.existsSync exists, then the 'path' module won't be imported unnecessarily.