Make boto3 an "extra" requirement

[DrGFreeman]

boto3 is currently defined in the install_requires parameter of setup in setup.py. This result in the boto3 and botocore packages being added to lambda layers built using AWS SAM tools. These two packages use about 60 MB of layer storage space, a significant fraction of the AWS lambda layer size limit of 250 MB, although they are not required to be installed in the lambda layer since they are included in the lambda runtime environment.

Moving boto3 to the extras_require parameter of the setup function would prevent the addition of boto3 and botocore to lambda layers while allowing their installation using the 'boto' extra option.

Update Installation section of README and docs to reflect the changes in installation options.