Return a hash with indifferent access rather than a Mash

[mboeh]

Problem

1.0.0 resolved #25, but Hashie::Mash is very different from a regular Hash. This causes issues with some existing code. In particular, symbolize_keys no longer works as expected.

Solution

Instead of converting the Setting hash into a Hashie::Mash, instead convert returned values that would be a Hash into a new SettingHash class. This is a normal Hash that has had some Hashie extensions added, notably IndifferentAccess.

Note that if this gem depended on ActiveSupport, I would just call .with_indifferent_access rather than messing with Hashie. It might be worth discussing changing the dependency.