Added check for grunt.file.isFile()

Fixes #10

Before:
![screen shot 2013-06-04 at 10 29 57](https://f.cloud.github.com/assets/678372/606052/099a7904-cd23-11e2-9422-446296ea3ef2.png)


After:
![screen shot 2013-06-04 at 15 09 52](https://f.cloud.github.com/assets/678372/606058/10dc12c2-cd23-11e2-96f4-5f7efa199982.png)


Tests:
![screen shot 2013-06-04 at 15 18 47](https://f.cloud.github.com/assets/678372/606062/18337de4-cd23-11e2-860d-982cc8ab3cec.png)