I used to do my global imports (i.e. imports that would be visible to all source files in my Xcode project) in the file AppName-prefix.pch
In Xcode 6 the project's .pch file not anymore generated automatically but if you want one then you can create a .pch file yourself. Do this as below;
Goto->File->New->File->(Under iOS)Other->PCH File->Next
After entering the name for the file and saving it, go to "Build Settings" and set the insert the value for "prefix header" as "YourProjectName/YourPCHFileName.pch". And if the "Precompile Prefix header" is set to "No" then change it to "Yes". Now you can put any file you want to import globally, but keeping everything here is not a good practice.