Implement GET /ping and GET /noop

[mcauser]

https://www.printnode.com/en/docs/api/curl#misc-ping
https://www.printnode.com/en/docs/api/curl#misc-noop

Related to PR #5 - auth error is currently putting an error message instead of throwing

[mcauser]

You can use Client.get()

@client.get('/ping')
@client.get('/noop')