Warn when an alias is used

[kdallmeyer-sr]

Since Baleen supports aliases, add validation code to warn if an alias is used. This assumes that underlying model (Avro, XML, etc) can handle aliases and that aliases were added specifically to the data description because it can happen. Therefore this is a warning and not an error.

[HawaiianSpork]

Good call that alias wasn't actually being used during validation. This pull request will change aliased fields from an error (if required) to a warning. Really the alias field should apply the attribute validation to the aliased attribute.