* Closes #8: Handling of optional values (#17)
* Refs #8: add optional and required fields to config node
* Refs #8: provide builder methods for optional and required access
* Refs #8: update readme
* Refs #8: Version to 1.1.1
* Fix badge link in readme
* Closes #19: Validation of optional fields throws errors when the field does not exist (#20)
* Refs #19: add tests
* Refs #19: skip None values in validation
* increment version
* Remove accidentally added dir (#21)
* Closes #24: Use proper python 3.7 for travis and cache pip dependencies (#27)
* Closes #25: Add new keys to the config (#28)
* Refs #25: add tests
* Refs #25: implement adding of new elements and add upsert flag
* update readme
* Refs #25: refactor output
* Closes #26: Merge with ENV variables (#29)
* Add documentation to config builder
* Refs #26: add test
* Refs #26: merge environment variables with config
* increment version to 1.1.3
* update readme
* Closes #34, closes #33: Enable config iterations and add to dict method (#36)
* reorganize config node tests
* Refs #34: add iteration functions
* Refs #34: add tests
* organize method order in config node
* Refs #34: add to dict method
* Refs #34: add to dict test
* Closes #32: Revise ENV merging (#37)
* Refs #32: move env merging to config node but keep builder method
* Refs #32: add and update tests
* Refs #32: update readme
* remove unused import
* change comment
* Closes #35: Switch to pytest and add flake 8 linting (#38)
* Refs #35: update readme and add flake8 and pytest
* Refs #35: add pytest ini
* Refs #35: add flake8 and update travis build
* Refs #35: fix linter errors
* Refs #35: remove init file contents
* Refs #35: change tests to pytest
* Version 1.1.4 (#39)
