Xcode6 generator

I've made a branch which contains code for a XCode generator that's tested in XCode 6.4. It's somewhat more 'advanced' than the existing xcode generator, in that it let the user create different types of XCode targets like app bundle, dylib, exe, framework etc. You can also build targets which depends on other targets. Hence, each time you build that target in XCode, it'll build the dependencies first.