Issue/62 orm adapter pattern

[simonireilly]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Related Issue	Fix algolia#62
Need Doc update	no

Describe your change

This change implements a Plug and Adapter for the ORM's supported by the algoliasearch-rails gem.

When the adapter is required we should invoke DatabaseAdapter.class_method_defined_within(*args) which is the Plug class.

This should pass this on to the required adapter. The required adapter must also implement this method e.g. DatabaseAdapter::RequiredAdapter.class_method_defined_within(*args).

Main

• DatabaseAdapter is a plug for various orm adapter classes
  • DatabaseAdapter::ActiveRecord
  • DatabaseAdapter::Sequel
  • DatabaseAdapter::Mongoid
• DatabaseAdapter implements logic for determining which ORM adapter should be used

Test

• Unit specs are added for all the new classes.
  • /spec/algoliasearch/database_adaper_spec.rb
  • /spec/algoliasearch/database_adapter/active_record_spec.rb
  • /spec/algoliasearch/database_adapter/sequel_spec.rb
  • /spec/algoliasearch/database_adapter/mongoid_spec.rb
• The ability to switch database in tests is added, to seperate unit spec concerns from integration concerns
  • spec/spec_helper.rb
• No new integration specs are added as all should continue to work as described

What problem is this fixing?

algolia#62 is correct, the codebase is very hard to read for new contributors and difficult in general because of various methods implemented for different ORM's.

This change will reduce the amount of complexity in the main algoliasearch-rails.rb and will also provide a separation of concerns for ORM's.