Added image url to asset
Added specs for image_url method in Asset class
Merge branch 'master' into image_url

Conflicts:
	spec/lib/contentful/management/asset_spec.rb
Added specs for image_url method from  Asset class
Added service objects for accessing associated collections
Merge pull request #20 from contentful/image_url

Image url
Naming and structure changes in methods factories
Fixed bug with turning string into class
Merge pull request #21 from contentful/modifying_nil_bugfix

Modifying nil bugfix
Merge with master