Rename internal structs

[rcmachado]

Internally, changelog current has the following structs:

• Changelog: the whole changelog data
• Version: represents each version
• ChangeType: Added, Changed, etc.
• ChangeList: messages for a given "section" (eg. Added).
• Item: each message

Some of the structs have pretty obvious names (like Version) but others (like ChangeList) could benefit of some more thinking.

[rcmachado]

Based on terminology adopted at keepachangelog.com website, ChangeType should be called Section.

For consistency, then ChangeList should be renamed to SectionList, SectionContents or similar (TBD).