handling symlink with '-L' option for copying

Related Issue & Code changes
* https://github.com/arturadib/shelljs/issues/69
* https://github.com/eisnerd/shelljs/commit/2e0ab9d9256e6b59ed128358eebc8b05d4bdd9d7

```
shelljs.cp('-r', src, dst);  // copy symbolic links as it is.
shelljs.cp('-rL', src, dst); // copy real files of symbolic link
```