Make RailsEventStoreActiveRecord indepdendent of Rails

[mostlyobvious]

ActiveRecord repository really depends on ActiveRecord. It is possible to use ActiveRecord without bringing in whole Rails.

This might involve rethinking how database migrations are provided -- at the moment we rely on Rails Generators. It may be possible to reduce this dependency to Thor.

[mostlyobvious]

(it might be easier make this move when #514 is merged)