Preserve scan filters when paging scans.
Fixes #47. The items argument for batch_get should not be mutated as an unexpected side effect
Merge pull request #46 from pifantastic/issue-44

Preserve scan filters when paging scans.
Merge branch 'devel' of github.com:jlafon/PynamoDB into devel

* 'devel' of github.com:jlafon/PynamoDB:
  Preserve scan filters when paging scans.
Fixes #45. The limit parameter should be decremented for each page of results returned from a scan operation.
Added example of conditional_operator not working in scan.
Added assertion