Changelog sections properly ignore duplicate entries.

When calling Section::addLine, the function scans through the current entries for that match, and will not add it if it exists already.
This is useful because with the transition towards a more automated approach of releases, the chance for duplicate entries is ever increasing.
GIT Commit Etiquette documentation.  Insightful.  Read it.
Add a "Security" type into the changelog system.

Since security is a crucial part of Core, (and should be a crucial part of any other system.....),
I figured it would make sense to have any security fixes stand out in the changelog.
Standardized the order of returning changelog entries.

This just helped remove some duplicate code.