Enhanced async implementation and ApiToken support

Hello,

please consider if you would like to reintegrate following changes:

- I have replaced the async CTP package by the current Async Targeting Package
- Additionally I have added the portable async http library for the async methods. The sync method have not changed for 3.5 Framework compatibility.
- For Support of Api Token and SecureString in Passwords I have added a Credentials Interface and two implementations for Username / Password and ApiToken
- The URL for zendesk can now by set simpler by just setting the subdomain name

Thanks,
Frank