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.

It also clarifies the Cocoapods part for people new to Cocoapods, and links to the sample app for examples on how to use the generated library.

Let's fix this before everybody hates me.