Generate targets with same build settings as Xcode (related to #164)

With the new version of the toolchain and Xcode, there were introduced new target types (iOS Framework) and some new build settings (i.e. if the target uses Swift.). We have to ensure that the generated targets build settings match those Xcode will create. This PR is related to #164.

Therefore I created a sample project, with a target for each type, dumped their build settings to xcconfig files, built an integration spec, and changed the constants so that the build settings will match the parsed xcconfig files.

~~Currently there will fail some specs, where we might need to discuss, how Xcodeproj should behave.~~

## ToDos

For easier code review I opened [a PR on my fork](/mrackwitz/Xcodeproj/pull/1) for these points.

* [x] Dump build settings in a Xcode version specific directory 
* [x] Deserialize xcconfigs on demand for `ProjectHelper::common_build_settings` instead of manually decompose them in the `Constants`
* [x] Move dumped build settings in another directory, because they will be used for more then being spec fixtures
* [x] Edit the [gemspec](/CocoaPods/Xcodeproj/blob/master/xcodeproj.gemspec#L19) to preserve the files in the packaged gem
* [x] Allow to specify the Xcode version as an argument for `ProjectHelper::common_build_settings`, use the current given by default
* [x] Update for Xcode 6.1
* [x] Let handle CocoaPods the build setting `DNS_BLOCK_ASSERTIONS=1` for Release configuration, if needed, as it is not included by Xcode by default anymore and so should not be included in targets created with Xcodeproj
* [x] Add an entry to the CHANGELOG.md
You can use `define_singleton_method` for this.