Push up sys.exit to bin/aws

The clidriver class now only returns RCs, and
bin/aws actually calls sys.exit.
Remove unused imports in clidriver
Handle empty row gracefully
Add automatic pagination support to CLI

I've added a TODO that we need to http responses for paginated
output but I think the plugin refactoring will address that.

The interface is written in such a way to allow for streaming
formatter responses, but the built in formatters all buffer their
objects. Eventually we should add streaming output formatters but
for formats like json we'll need a streaming json parser.
Merge branch 'pagination' into develop

* pagination:
  Add automatic pagination support to CLI
  Handle empty row gracefully
  Remove unused imports in clidriver
  Push up sys.exit to bin/aws