Version 0.5.0

Bump the version to 0.5.0 and update the CHANGELOG to reflect the
changes introduced in e22226ad19ca09fd017f3c03121dad3a7446db66.

Releasing this now so that the pull request alphagov/vcloud-launcher#35
can be merged as it depends on these changes.
Merge pull request #71 from alphagov/71583392-bump-version

Version 0.5.0