Add YAML support (Fixes #8)

I decided to look through some issues and noticed that #8 was standing around without any updates to the dead (until I have time to develop it or someone else takes over the project) [librarian-vagrant](https://github.com/ipwnstuff/librarian-vagrant) project that was spun off to improve bindler.

Here's a possible solution from [my comment on #8](https://github.com/fgrehm/bindler/issues/8#issuecomment-26223366).

Since [YAML is a superset of JSON](http://yaml.org/spec/1.2/spec.html#id2759572), we can use parse JSON files as if they were YAML.
```ruby
JSON.parse(json) == YAML.parse(json).to_ruby
```