Refactor code to be more maintainable

[jeffsw]

Following the addition of initial layer support, the code is pretty ugly. I'll refactor it a bit later. Some plans:

• Create a superclass with methods common to AwsLambdaLayer and NewAwsLambdaLayerZip
• When publishing an NewAwsLambdaLayerZip, return an AwsLambdaLayer object since we can do so
• separate out a lot of utility functions into their own source file importable by all the others
• move functions like aws_lambda_update and s3_upload to become methods of a NewAwsLambdaZip or whatever
• move things from __init__.py