Bugfix/package persistency ios

This branch contains:
* Bugfix for changes made to iOS 8 regarding the sandbox: Package were not correctly reinstantiated upon startup. Package URLs are now stored relative to the caches folder.
* Cleaned up Mac target warnings
* Refactorings for packages: 
  * Packages are persisted more
  * Removed some manager methods to prevent issues with uninstallable packages due to inferred resolution and OS. A package downloaded from a custom URL has to be identified explicitely by its name, resolution and OS.
  * Mac target uses iOS packages
  * Package installer will delete existing packages with the same identifier, preventing uninstallable packages due to a loss of persistency data