Split up aws_assume_role.py into a few files to make it easier to work on

I am looking at providing a PR to break up the aws_assume_role.py file into a few files grouped by their purpose such as 
* saml.py
* console.py
* onelogin.py
* cache.py

I am working to customise the tool to make it a bit easier for our use case and it would be great to separate out some of the logic. 