Allow the specification the Xcode target version

The Podfile should support the following syntax:

``` ruby
xcode_version = 5
xcode_version = 6
```

This would allow to merge work in progress support for new Xcode versions in Xcodeproj and not force deviation from the build settings used by the last version of Xcode to keep support for the previous version.

Related: https://github.com/CocoaPods/Xcodeproj/pull/166
