Update to WordPress 3.6

Remove some STRICT ERRORS
Undefined offset
Format code
Undefined variable
Drop PHP4 support
Update changelog
Update comment section to confirm with workflow
Update version number
Version bump 3.8.0-rc.1
Version bump 3.8.0
Merge branch 'release/3.8.0'
Merge tag '3.8.0' into develop

3.8.0
Preparation for new development cycle after release 3.8.0