Fix Nested Locale handling

[dlitvakb]

Currently we have two ways of handling Locales

1. Entry#fields accept a Locale as a parameter and returns localized versions of the resources (Sync API only) Parse nested locales in AssetFields #66
2. Each field on an Entry has a Hash with locales upon request with locale: "*" (contentful_middleman multiple locale support, it was needed that by default each entry would have locales readily available) Add support for multiple locales on Entries #72

The second way was introduced to give support to contentful_middleman for multiple locales.

We need to standardize it so there's only one way to deal with Locales. We also need to make sure that the way we choose to not make default still works afterwards to avoid breaking production code.

The reason this is a bit tricky is because each field can be localized separately in the CDA API and we need the Content Type definition to check each field separately.

Currently the nested_locale_fields functionality works for the Sync API because every field, independently of configuration will return a Locale.