use ActiveSupport::HashWithIndifferentAccess for field mappings

[jasonayre]

To prevent stuff like below from happening:

https://www.dropbox.com/s/32yu3nnrd8pooqa/Screenshot%202015-11-12%2010.04.52.png?dl=0

Which prevents save/create of models if the user makes mistake of specifying field names as strings instead of symbols.

It will require activesupport as a dependency, however since most people using this gem will already be using rails, I don't see any reason not to add that dependency. Also if you add active model as a dependency as per #95 , dependency will be met anyways. Im happy to PR if you will accept.

[eloyesp]

I don't like the idea of adding Active Support, it's true that ActiveModel depend on it, but this is like a bug to me, I don't want to promote it to feature.

To prevent that kind of issue we could raise an ArgumentError when defining the field or use .to_sym to enforce the use of symbols. Do you like that idea?