documentation on commit messages

per request from @fanquake
- expand existing content in CONTRIBUTING.md and break out into section
- add section to HACKING.md
add missing ".undent"s
Merge pull request #2509 from rolandwalker/caveats_formatting

add missing `.undent`s
Merge pull request #2511 from rolandwalker/aptana_studio_version

Downgrade Aptana Studio to version 3.4.2
lightpaper 0.8.7
Merge pull request #2475 from rolandwalker/doc_commits

documentation on commit messages
Merge pull request #2516 from chulkilee/lightpaper-0.8.7

lightpaper 0.8.7
move squashing doc from HACKING to CONTRIBUTING.md