Allow YAML aliases in ActiveYaml

Allow YAML aliases in ActiveYaml.

ActiveYaml will filter any arrays with keys whose first character is the '/' forward slash.
This allows users to create root elements in the YAML in which aliases can be stored nicely.
Tested with both array and key rows.

Aliases can be used like follows:

     - /aliases:
      soda_flavor: &soda_flavor
        sweet
      soda_price: &soda_price
        1.0

    - id: 1
      name: Coke
      flavor: *soda_flavor
      price: *soda_price