Signature validation in AleHu client and promising tests

So far, signature validation has only been done by the AleHu server. However, with the latest release (AleHu 20140315.zip) this deficiency was solved and signature validation is now done by the receiving AleHu client, too.

Further testing has been done lately and the results seem to be rather promising. Adding an appropriate RequestRetryHandler to the HttpClient logic was a major leap forward in client stability.