Add CocoaPods support

[radex]

This resolves #1.

There's just one problem before this can just work: I named the Pod "SwiftProtobufRuntime", but the generated .pb.swift files do import Protobuf — they think this library should be called just "Protobuf".

There's a few possible solutions:

1. I could rename the Pod to "Protobuf". However, there already exists a Pod named Protobuf. If Apple isn't interested in pushing the pod to the CocoaPods trunk, this would work fine. But it would still be confusing to users, and if someone has an existing iOS app that uses the Protobuf pod, they couldn't also use this library.
2. An option could be added to swift-protobuf-plugin to customize the generated import, so people using CocoaPods or already using Protobuf in their apps (with other libraries) could resolve the naming conflict
3. The runtime module name could be changed to something unambiguous, like ProtobufSwift or ProtobufRuntime for everyone.
4. I think it should be possible to customize the Podspec so that a module name "Protobuf" is generated, while the Pod itself is still called "SwiftProtobufRuntime" — but I'm still working out how to do that exactly ;)

WDYT?

[jcanizales]

My 2c:

• Option 1 would indeed be confusing :)
• Option 4 is possible in ObjC and C with the podspec attribute module_name. I haven't tried it with Swift sources, but I imagine it works the same. It's what I did for gRPC-Core.podspec and BoringSSL.podspec, that needed to generate frameworks called grpc and openssl respectively. It still would make it impossible to use the Swift and ObjC runtimes in the same app, which is unfortunate.
• I thought option 2 would face the same problem as the ObjC version: Because the runtime includes generated protos (empty.proto etc.), that fixes the name of the module. But none of those generated files import Protobuf! Maybe they were hand-edited after generation?

My opinion is ProtobufSwift would be the simplest option. We already have libraries named like RxSwift, so it wouldn't be an outlier.

[radex]

Okay, I added module_name option to the Podspec (as @jcanizales and a few others suggested), so the Pod generates a module name "Protobuf" and it just works.

I'd still suggest considering options (2) and (3), but the PR is mergeable as it is ;)

[tbkka]

@jcanizales : The generated files in the runtime library are in fact edited to remove the import statements. See https://github.com/apple/swift-protobuf-runtime/blob/master/Makefile#L95

[tbkka]

I'll see about renaming the module to "SwiftProtobuf". That will be a little disruptive in the short-term, but seems the best long-term fix. I'd like to do that before we merge this, if that's okay. I'll get to it soon.

[tbkka]

I just committed the name change; the module is now called 'SwiftProtobuf'.

Please update your merge request accordingly and I'll merge it.

Thanks!

[radex]

Thank you — 'SwiftProtobuf' makes a ton of sense!

I updated the Podspec and Readme — should be good to merge!

[tbkka]

I've merged this, but we may need to update it after Issue #8 gets finished.