Aggregate with key is None

[bmaggard]

This PR has the objective of...

Changes

Allow aggregation over the entire table (without a key)

Checklist

Checklist for for pull requests including new code and/or changes to existing code...

• [x] Code
  • [x] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [x] Docstrings include notes for any changes to API or behaviour
  • [x] All changes documented in docs/changes.rst
• [ ] Testing
  • [ ] (Optional) Tested local against remote servers
  • [ ] Travis CI passes (unit tests run under Linux)
  • [ ] AppVeyor CI passes (unit tests run under Windows)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [ ] Changes
  • [ ] (Optional) Just a proof of concept
  • [ ] (Optional) Work in progress
  • [ ] Ready to review
  • [ ] Ready to merge

[bmaggard]

Not sure how this branch was behind