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Source prototype.
Refactoring.
User 固有の Source を生成する
SourceController 追加
Rename classes and methods.

Sources                 => Strage
UserSource              => Source
UserSource#source_class => Source#klass
Create Storage module.
Add 'storage_key' column to Source table.
Remove klass field from sources#new and sources#edit.
Remove klass column from sources table.
SourceController handles error which raises when storage does not exists.
Add Source.create
Modify Source schema.
Add mongoid gem.
Add mongoid-rspec gem.
Add Crawler model.
Storage key is generated automatically when storage is created.
Make spec independent of Storage::File.
Drop Storage::File and create Storage::Mongo.
Merge Storage and Storage::Mongo.