Unify paging params

* Remove built in paging params for paginated operations
* Add a ``--starting-token`` and a ``--max-items`` option for paginated operations.

Non paginated operations are left untouched.

If you want to try this out locally, you'll need to pull in boto/botocore#88
This will also fail the travis build as it's using botocore:develop.

cc @garnaat @toastdriven