Parse nested locales in AssetFields

[neonichu]

Fixes #63

[neonichu]

I have simply copied the magic from Contentful::Resource::Fields so that assets work the same way. I didn't fully understand why the split between the two was made, would obviously be nicer if both code paths would simply share this functionality.

[pxlpnk]

Suggestion:
We pull this:

contentful.rb/lib/contentful/resource/fields.rb

Lines 28 to 43 in 5ee811d

	def extract_fields_from_object!(object)
	@fields = {}
	if nested_locale_fields?
	object['fields'].each do |field_name, nested_child_object|
	nested_child_object.each do |object_locale, real_child_object|
	@fields[object_locale] ||= {}
	@fields[object_locale].merge! extract_from_object(
	{ field_name => real_child_object }, :fields
	)
	end
	end
	else
	@fields[locale] = extract_from_object object['fields'], :fields
	end
	end

into the resource.rb module and make it available on the fields.rb and assets_fields.rb modules?
With this we can avoid code duplication as that code on its own is already rather complex.
WDYT?

[neonichu]

@pxlpnk sounds good, will update the PR

[pxlpnk]

Looks good!