Integrate a build phase that checks if Manifest.lock is in sync

As discussed [on the mailing list](https://groups.google.com/forum/?fromgroups=#!topic/cocoapods/KzZ__kXpcmo).

This adds a build phase to the Applications target that checks if `Pods/Manifest.lock` is in sync with `Podfile.lock`. This helps to inform developers of updating their Pods when the `Pods` directory is not tracked by version control.