Organize changelog

Given the number of changes since 0.25.1, categorize the changes into
three types: new features, changes, and bug fixes.

This will hopefully prove more useful to those reading the changelog.

Change-Id: I68366fe86b8be3ca2abc6b3b494ccef9806d656b
Reviewed-on: http://gerrit.causes.com/40986
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>