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

- Remove the launch screen.
- Add a label explaining where to look for the code and results.
- Delete unnecessary boilerplate code.

The tests are going to use it too

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.

Because the sample protos were all in a flat directory, I hadn't noticed the issue.