Petl

Issues & pull requests resolved in this release:
- https://github.com/alimanfoo/petl/issues?q=milestone%3A0.26

Contributors:
- andrewakim
- rs
- podpearson

Thanks also to imazor and cons0l3.

Minor release adding missing petl.io to setup.py (0.25 is broken).

This release includes a number of improvements and bug fixes, see the [full list of issues resolved](https://github.com/alimanfoo/petl/issues?q=milestone%3A0.25+is%3Aclosed) for details.

Contributors to this release:
- rogerkwoodley
- alimanfoo

Please note this release contains some backwards-incompatible changes:
- The functions `petl.valuecounts()` and `petl.aggregate()` have changed their behaviour slightly, such that the fields in the output table may be different from previous versions. If a single field is used as the `key` argument then that field name will be used instead of "key" in the output table header. If a compound key is used, all key fields will be output as separate fields.
- The variable `petl.transform.defaultbuffersize` has moved to `petl.transform.sorts.defaultbuffersize`.

Note also that the `petl.transform` package has been refactored to break what was a very large module up into smaller pieces. Similar work is planned for the `petl.io` module in a subsequent release.

A full list of all changes is available at https://github.com/alimanfoo/petl/issues?milestone=23&state=closed

List of changes: https://github.com/alimanfoo/petl/issues?labels=&milestone=22&page=1&state=closed