Store the application data in yaml files.

In order to simplify the addition of new contents the classes, races
and background files are store in yamls. This way is clear that type
of content is data. On this way the models are free to work with
logic and reduce the noise with data storage.
Reduce code complexity with rails useful helpers.

The intention of the seeds directory is that will be
only stored yaml files. With rails helpers we can
reduce the yaml extesion checker and assume that
files always will be right.
Change Repository.loa type argument to String.

Since it is in our control, we can pass String and
avoid unecessary Symbol->String convertion.
Convert symbol stored data to string.

The stored yaml data should be simple. Not worry about
symbols.
The contents should be sorted by its slug.

I was surprised about travis ci give the files
in a different order. To make sure the contents
has a specfic order, we are making sure it
ordering the items by its slug.