Fix for Guard ~> 2.0.0

* Change the directory name for Guard template searching.
* Allow safe_yaml gem to parse symbols in `Guard::Guard` namespace. It's form [safe_yaml issue].(https://github.com/dtao/safe_yaml/issues/10)
* Add some spec in `test/lib/guard/jekyll-plus_test.rb`.

I haven't touch version number and the dependency of Guard. Also I add the test for the change, if you like it in your favorite way (e.g.:`rspec` or `assets`), please let me know and I'll love to change it for you.