Re-enable all release actions

After today's successful release using this playbook I can now
re-enable the commented actions with some certainty.

I have also added a `release_type` variable that needs to be set
using the command line. This may either be major, minor or patch
and indicates which type of release this is.
Add Unreleased CHANGELOG section

To keep track of tasks that have been executed I have added
a section 'Unreleased' to the CHANGELOG where, with each PR,
we can add what has been changed so that when we make a new
release we can replace the Unreleased with a date and version
number.
Updated CHANGELOG to reflect template change
Merge branch 'develop' of github.com:phpDocumentor/phpDocumentor2 into develop