Fix infinite recursion caused by dir symlink

`stat.isDirectory()` (https://github.com/remy/nodemon/blob/master/lib/monitor/watch.js#L112) returns true for symlinks pointing to a directory, potentially resulting in an infinite `watch()` recurse.

To reproduce:
```
mkdir test && cd test
touch test.js
ln -s . test
nodemon ./test.js
```
also
```
ln -s . ./node_modules/test
```
node CPU usage % should be very high now.