Update to work with Ethel changes

- add alternate file patterns
- move ethel/sequel folder to ethel/adapters/sequel
- move ethel/sources/sequel.rb to ethel/adapters/sequel/reader.rb and
  rewrite to inherit from Ethel::Reader
- move ethel/targets/sequel.rb to ethel/adapters/sequel/writer.rb and
  rewrite to inherit from Ethel::Writer
- move ethel/sequel/version.rb to ethel/adapters/sequel/version.rb