Update changelog
Rename tests

- test_pydicti => test_dicti: this module will specifically run tests
  for the `dicti` class.
- test_compat => test_common: common base class for all specific tests
Extend tests massively
Simplify testing sequence

It would be pretty nice show off the extra_requires feature, but I think
in fact it only causes confusion and looks ugly.
Mention improved test coverage in CHANGELOG
Merge branch 'improve-test-coverage'