Verison 3.0.0

[bbayles]

This PR is for version 3.0.0. The changes are:

• boto3 client handling is now simplified. The region_name, profile_name, and boto_client_kwargs have been dropped. You can set the usual AWS environment variables to customize client creation, or pass in clients to the constructors yourself.
• Support for custom formats on CloudWatch logs has been added. By default they will be automatically detected using the ec2 DescribeFlowLogs API endpoint. You can also pass them in manually with the fields keyword in the library, or the --fields switch on the command line.
• The reader classes now keep track of the number of bytes_processed.

[mrg29]

abridged/edited conversation from Slack:

me:
On this do we need to worry about characters that are more than 1 byte in utf-8?

bbayles:
At the moment it's all ASCII.