The examples under <repo_root>/examples now rely on the released
versions. Those under src/objective-c/examples, as well as the tests,
rely on protoc and the plugin as compiled from head.

Still needed:

- Make it build protoc if it’s not in the root (i.e. if we're developing locally)
- Put the gRPC plugin in the GitHub release page, and add a podspec for it that does the same

This works around the Cocoapods' linter header_mapping_dir restrictions.
Incidentally, it also removes the need to set build settings in the user
target.

Removed the silencing for incompatible-pointer-types
Removed unused objects
Fixed format issues

Update Podfile for CocoaPods 1.0 changes requiring more fields and the removal of link_with in podspec.

* Reference discussion: https://github.com/grpc/grpc/pull/4108#discussion-diff-44494393

For local running one can still tests AllTests, but for C.I. one can
test the 4 of them in parallel (and, importantly, deactivate the remote
interop tests until we fix them on Travis; while keeping the rest).

I had to make the remote tests a subclass of an abstract InteropTests
(like InteropTestsLocalSSL and InteropTestsLocalCleartext), and then
deactivate the abstract InteropTests on the 4 schemes that use it.