[builder] Do not try to compile completely empty .strings files.

Fixes http://hipbyte.myjetbrains.com/youtrack/issue/RM-483
[builder] Quote framework search paths to guard against spaces.
move #vendor_project to xcode config
move the `rake clear’ logic to platform-specific config files
move versionized_build_dir from config to xcodeconfig
move app.version from config to xcodeconfig
2.28
should not drop "10.10" from supported sdk version list

This should allow to use "10.10" sdk on Yosemite
accept the lower SDK version in `app.deployment_target' on Yosemite
2.29
update NEWS
initial support for extensions