only try to serve regular files

I added a check if the requested file is actually a regular file. Without this check, connect-jade-static tried to serve requests for directory indices as well. This resulted in errors of the form:

```
Error: EISDIR, illegal operation on a directory
   at Object.fs.readSync (fs.js:476:19)
   at Object.fs.readFileSync (fs.js:310:28) 
   at Object.exports.renderFile (/home/adrian/....y/node_modules/jade/lib/jade.js:340:10)
   at Object.exports.renderFile (/home/adrian/...../node_modules/jade/lib/jade.js:326:21)
   at isExists (/home/adrian/Documents/..../node_modules/connect-jade-static/index.js:31:12)
   at Object.cb [as oncomplete] (fs.js:168:19) 
```
I agree, we can get rid of this additional check. Maybe I will fix it, as soon as I run into trouble with regards to performance.