Share an event emitter between session and clidriver
Use session object to emit events in the CLI

At this point, all the mechanics of emitting events
is done in botocore, though the CLI does share an
emitter with the session object.
Remove before/after-operation in clidriver

These events have been moved into botocore and happen
much closer to when the actual operation is called.
Merge branch 'page-events' of git://github.com/jamesls/aws-cli into jamesls-page-events