[RFR] Clarify Map logic, allow nested fields

[fzaninotto]

Currently, callbacks registered on fields with map() are applied to the REST response to turn it into an Entry (when fetching results) AND to the entries to turn them into a response (when pushing modifications). It doesn't make sense, and prevents some new features, such as deeply nested fields (refs. #525).

This PR fixes that. Now you can do things like:

listView.fields([
    nga.field('title'),
    nga.field('author.name')
]);

to map REST responses looking like:

[
  {
    "title": "War and Peace",
    "author": {
      "name": "Leo Tolstoi"
    }
  },
  {
    "title": "Anna Karenina",
    "author": {
      "name": "Leo Tolstoi"
    }
  }
]

Depends on marmelab/admin-config#20

[fzaninotto]

Switching to RFR!

[jeromemacias]

admin-config 0.2.5 version had been published.

[fzaninotto]

Switched dependency to fixed version