CocoaPods 0.17

This patch performs the following changes

- Add support for CocoaPods 0.17
- Ask to CocoaPods if an installation is needed providing support for teams which do not keep the Pods folder under version control.
- Don't make assumptions about the paths of the files generated by CocoaPods (his assumption would have caused problems with the Sandbox folder reorganization).

```console
$ bundle exec rake
[..snip…]
10 specifications (14 requirements), 0 failures, 0 errors
```

I've not tested an actual installation.

@alloy, @lrz