download_to_fileobj() implementation per boto/boto#1941
Unit tests for new methods added in boto/boto#1960
Clarifying documentation for CloudWatchConnection.list_metrics().

- Multiple values for the same dimension are not supported by the API
- Max 10 dimensions allowed
Updated Autoscaling for the latest API changes.
Release notes for 2.22.0.
Bumped to 2.22.0!
Added Autoscale Tag class to API docs
Merge pull request #1962 from oxtopus/documentation-fix-for-list_metrics

Clarifying documentation for CloudWatchConnection.list_metrics(). Fixes #1962.
Merge pull request #1964 from micahhausler/develop

Added Autoscale Tag class to API docs. Fixes #1964.
Merge pull request #1960 from oxtopus/issue-1941-download_to_fileobj

Glacier download_to_fileobj implementation. Fixes #1960.
Set security token via environment or config file

Support AWS_SECURITY_TOKEN environment variable and
aws_security_token in the config file.  Unit tests and doc change
included.  Ignored updating core/ as I assume core is abandoned in
favor of the separate botocore project.