Update client library to support entitlements

[lsfernandes92]

This PR aims to:

• Adds CustomerPermission, Entitlement and ExternalSubscription models.
• Adds GET v2/accounts/{account_id}/entitlements endpoint to this client library.

Sample code request:

begin
  account = Recurly::Account.find('<ACCOUNT_CODE>')
  account.entitlements.find_each do |entitlement|
    puts "Entitlement: #{entitlement}"
  end
rescue Recurly::Error => e
  e.inspect
end