[xctool] reduce build-tests overhead by invoking xcodebuild only once
Merge branch 'master' into yiding/reduce-build-test-overhead

Conflicts:
	xctool/xctool.xcodeproj/project.pbxproj
	xctool/xctool/XcodeSubjectInfo.m
Merge pull request #43 from facebook/yiding/reduce-build-test-overhead

[xctool] reduce build-tests overhead by invoking xcodebuild only once