Add request adapters

[felipeelias]

This introduces a new module called Request. The idea is to remove duplication in the API class by moving request related methods to their own class and allow the user to create its own requester class:

class MyRequester < Embedly::Request::Base
  def get(uri, options = {})
    # performs the request
  end
end

This class is responsible to make the request. The user can register the new class in configuration and use it as follow:

Embedly.configuration.add_requester :my_requester do |api|
  MyRequester.new(api)
end

Embedly.configure do |config|
  config.request_with :my_requester
end

After that, all api requests are made with the new request class. This allows users to user faraday gem for example, without messing up with the api class.

As an example, you can check that the _do_typhoeus_call method was moved to Embedly::Typhoeus::Request.