Change the default script for Objective-C to xctool
Shellescape the xctool workspace, scheme and project
Prefix Objective-C config vars with xcode_

This should make the script more backwards compatible, as we used to
have xcode_sdk and xcode_scheme.
Merge remote-tracking branch 'origin/master' into hh-add-xctool-support

Conflicts:
	spec/templates/header.sh
Check that project or workspace is set
Merge pull request #109 from travis-ci/hh-add-xctool-support

Use xctool as the default OS X build script