[Sandbox] Symlink vendored framework headers into the public headers dir.

So, this is the fix for #1992, but now that I’ve finished it and have been reviewing the various tickets related to it, I’m unsure if it’s really still needed or a good idea to link vendors framework headers into the `Headers/Public` dir at all.

* #1992 is _probably_ about preserving the files, including the header files, this already works nowadays.
* The reason for #2382 is completely unclear to me. @neonichu I remember talking to you when you filed this, do you still remember the exact reason for it?
* The issue of frameworks in local pods (https://github.com/CocoaPods/CocoaPods/issues/1992#issuecomment-41895130) has already been fixed by 94e3b3a7447dd2b5e088e17ccb1c66b581415ed6.

@orta @kyle Does CocoaDocs handle headers of vendored frameworks correctly or would there be a benefit for CocoaDocs to at least merge [the file accessor for framework headers](https://github.com/CocoaPods/CocoaPods/commit/c0de45e36205d946a58972046ac8254f219b72f0#diff-a1bd48eb509ce3365557319fdda59f25R115), which correctly handles symlinks? (i.e. so without the code that creates the symlinks in `Headers/Public`.)