Implement Mamiya::Storages::Abstract, S3
Use .find instead of #applications

- before

``` ruby
storage_without_app = Mamiya::Storages::S3.new(bucket: 'packages', access_key_id: '...', secret_access_key: '...')
p storage_without_app.applications['myapp'] == \
  Mamiya::Storages::S3.new(bucket: 'packages', access_key_id: '...', secret_access_key: '...', application: 'myapp')
```

- after

``` ruby
applications = Mamiya::Storages::S3.find(bucket: 'packages', access_key_id: '...', secret_access_key: '...')
p applications['myapp'] == \
  Mamiya::Storages::S3.new(bucket: 'packages', access_key_id: '...', secret_access_key: '...', application: 'myapp') #=> true
```