Support namespaced keys

Adds support for namespaced keys in #data_for, e.g., "user/valid",
"user/name_missing", etc. Each namespace loads a different YAML file in
the data directory. If the key given is not a string, or it does not
contain "/", then the previous behavior is preserved.
Fix failing 1.8.7 travis builds by sorting expected hash keys
Merge pull request #12 from bak/namespaced_data

Support namespaced keys