`/_source` mapping is optional, .st committed to .js dir by default
CHANGELOG update.
Minor fixex to CHANGELOG and API-CHANGES related to 0.12.3.
Merge pull request #876 from herby/fix-0-12-3-metadata

Minor fixes to CHANGELOG and API-CHANGES related to 0.12.3.
Merge pull request #875 from herby/underscore-source-optional

Underscore source optional
Moves all st files in .js
Renames js/ to src/
- Renames cli/js to cli/src
- updates cli/index.html
- updates cli/support/setversion.sh