Add CocoaPods support

[philtre]

I've added a Podspec file to allow using Aphid in iOS and macOS projects.
When adding Aphid to your project, also add BlueSocket and BlueSSLService pods:

pod "BlueSocket"
pod "BlueSSLService"
pod "Aphid"

Or, to use it in another library, add Aphid as a dependency to your Podspec:

s.dependency "Aphid"

To use Aphid in your code, import both BlueSocket and Aphid:

import Socket
import Aphid

I've been able to successfully build Aphid using CocoaPods on both iOS and macOS.