Introduced grpc_byte_buffer_init instead. It's now the user's responsibility to
manage memory.

In library projects (cf. app projects) the main bundle is nil. `NSBundle+bundleForClass:` works in both types of projects.

Also makes the library load the certificates only once.

On account of the change in the podspec structure.

- Moves them to the root of the repo.
- Makes them pass lint (except for the release tag warning).
- Includes the script to rename time.h and string.h in the spec.
- Fixes the #includes in the wrapper files.