Update CHANGELOG for 0.4.0

To include other changes since 0.3.0. I've renamed "maint" to "API changes"
following a discussion about it not reflecting the contents. New name was
cribbed from python-requests.
Release 0.4.0
Merge pull request #62 from alphagov/70452106-release_0_4_0

[#70452106] Release 0.4.0