Update Cache docs for recent changes.
Update logging docs for config changes.
Update for configs not being modifiable.
Update changelog.
Fix typos.
Update docs for email config.
Update migration guide for various changes.
Fix/add docs for dropping configs.
Fix mistakes add namespaces.
Update migration guide more & fix mistake.
Merge pull request #699 from cakephp/3.0-config

3.0 config