Support AWS authentication method

[stevefranks]

It looks like AWS isn't fully supported as an authentication method yet within hvac. I'm going to work on adding this. I hope to have a PR ready for review shortly.

https://www.vaultproject.io/api/auth/aws/index.html

[jeffwecan]

@stevefranks Sounds good and thanks! If you have any questions at all regarding the best way to implement this contribution, feel free to lodge those queries here or in the hvac gitter channel at: https://gitter.im/hvac/community

[jeffwecan]

Also for what it's worth I went down this route way back when and pushed up that work that I was able to complete along those lines now at: https://github.com/jeffwecan/hvac/blob/1573406a231bb31c3779aa2722eeee130fc51247/hvac/api/aws.py

That version of the class doesn't incorporate the current auth class baseclass / directory structure since it was last touched before those elements were finalized. However it may still give you a good starting point for many of the AWS auth methods that remain to be implemented.