Release 0.4.3

• Add more channel arguments (@SebastianThiebaud)
• Add casts to fix warnings about ints used as void *. (@timburks)
• README updates (@rebello95)
• Fix memory leaks (@MrMage)
• Avoid a try! call (@MrMage)
• Make the test stubs thread safe (@MrMage)
• Leave zlib dependency only for non-Apple platforms (@MrMage)
• Add CocoaPods-based sample (@timburks)
• Fix broken link (@koichi.tanaka)
• Spin up dedicated queues for some operations (@MrMage)
• Dispatch the actual request handlers on new threads (@MrMage)
• Remove several now-unused DispatchQueue arguments (@MrMage)
• Make the server dispatch each request handler on its own thread (@MrMage)
• Streamline the interface of the server-side response handling code (@MrMage)
• Fix several completion queue leaks in tests (@MrMage)
• Add server/client codegen options to protoc (@rebello95)
• Add docs for protoc parameter usage (@rebello95)