Provide automatic fallbacks when creating auth object for AWS

[linusthe3rd]

Looking at auth.go, the three methods of retrieving the access keys to make API calls to AWS are available:

• the consumer provides the access keys
• get the access keys from environment variables
• get the access keys from the local metadata server

The problem with this is that, as a consumer, I need to add logic to handle falling back to one of the above methods if a previous one fails (provided by consumer -> checked env -> check metadata server). Comparing this code to goamz's GetAuth method, goamz provides a nice wrapper method to encapsulate this fallback logic so that the code will seamlessly work in all three scenarios.

It would be nice to have a method like this for go-kinesis.

[le0pard]

Right now I don't have time to add this, but ready accept PR for this improvement.