add and remove photos to an album

-update views
* add photo partial
* add photo_form partial
* add error_message partial
* showing album photos in its show page

-update photo controller to be created under album
* create
* destroy
* documentation:
http://guides.rubyonrails.org/association_basics.html#has_many-association-reference

-nested routing for album and photos
* documentation:
http://guides.rubyonrails.org/routing.html#nested-resources