Improve internal and external interface

The gem was decomposed into several files and moved into the TrepScore::Services namespace. 

We also gain a few new features:

- required vs optional parameters
- retrieving data for a given time period
- automatic enforcing of required schema attributes
- custom validations for schema data
- connection test method to confirm provided data against the 3rd party service

```ruby
module TrepScore
  module Services
    class Pipedrive < Service
      category :crm

      required do
        string :token
      end

      def call(period)
        client = ::Pipedrive::Client.new(api_token: data['token'])
        client.metrics(period)
      end

      def validate
        unless data['token'].length == 28
          raise_config_error('Token appears to be invalid')
        end
      end

      def test
        # Check Credentials
      end
    end
  end
end
```
Prefer `Object#is_a?` over `Object#kind_of?`.