Add ability to change storage_method, Change default storage method

[westonganger]

Add alternative storage column options to store snapshots.metadata and snapshot_items.object

• Change default from serialized_yaml to serialized_json
• Add support for native_json

More Info:

• Solves #15
• Solves #31
• Builds upon the changes in #25

[westonganger]

@oskargargas do you want to try the latest changes to master and see if that is sufficient for your purposes.

You would need to set

ActiveSnapshot.config do |config|
  #config.storage_method = "serialized_json" # default, for text column
  #config.storage_method = "serialized_yaml" # for text column
  config.storage_method = "native_json" # for json/jsonb column
end