Add Cocoapods podspec generation. (#3)

Motivation:

A number of users have asked for the ability to install
NIOTransportServices via Cocoapods. This is an entirely reasonable
request, so let's do it.

Modifications:

Added a script that can generate, and optionally upload, a specfile
for NIOTransportServices.

Result:

Users can use cocoapods if they prefer.

scripts/build_podspec.sh

@@ -0,0 +1,81 @@
+#!/bin/bash
+##===----------------------------------------------------------------------===##
+##
+## This source file is part of the SwiftNIO open source project
+##
+## Copyright (c) 2017-2018 Apple Inc. and the SwiftNIO project authors
+## Licensed under Apache License v2.0
+##
+## See LICENSE.txt for license information
+## See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+##
+## SPDX-License-Identifier: Apache-2.0
+##
+##===----------------------------------------------------------------------===##
+
+set -eu
+
+function usage() {
+  echo "$0 [-u] version"
+  echo
+  echo "OPTIONS:"
+  echo "  -u: Additionally upload the podspec"
+}
+
+upload=false
+while getopts ":u" opt; do
+  case $opt in
+    u)
+      upload=true
+      ;;
+    \?)
+      usage
+      exit 1
+      ;;
+  esac
+done
+shift "$((OPTIND-1))"
+
+if [[ $# -eq 0 ]]; then
+  echo "Must provide target version"
+  exit 1
+fi
+
+version=$1
+NAME="SwiftNIOTransportServices"
+
+here="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+tmpdir=$(mktemp -d /tmp/.build_podspecsXXXXXX)
+echo "Building podspec in $tmpdir"
+
+cat > "${tmpdir}/${NAME}.podspec" <<- EOF
+Pod::Spec.new do |s|
+  s.name = '$NAME'
+  s.version = '$version'
+  s.license = { :type => 'Apache 2.0', :file => 'LICENSE.txt' }
+  s.summary = 'Extensions for SwiftNIO to support Apple platforms as first-class citizens.'
+  s.homepage = 'https://github.com/apple/swift-nio-transport-services'
+  s.author = 'Apple Inc.'
+  s.source = { :git => 'https://github.com/apple/swift-nio-transport-services.git', :tag => s.version.to_s }
+  s.documentation_url = 'https://github.com/apple/swift-nio-transport-services'
+  s.module_name = 'NIOTransportServices'
+
+  s.swift_version = '4.1'
+  s.cocoapods_version = '>=1.1.0'
+  s.ios.deployment_target = '12.0'
+  s.osx.deployment_target = '10.14'
+  s.tvos.deployment_target = '12.0'
+
+  s.source_files = 'Sources/NIOTransportServices/**/*.swift'
+  s.dependency 'SwiftNIO', '~> 1.8'
+  s.dependency 'SwiftNIOFoundationCompat', '~> 1.8'
+  s.dependency 'SwiftNIOConcurrencyHelpers', '~> 1.8'
+  s.dependency 'SwiftNIOTLS', '~> 1.8'
+end
+EOF
+
+if $upload; then
+  echo "Uploading ${tmpdir}/${NAME}.podspec"
+  pod trunk push "${tmpdir}/${NAME}.podspec"
+fi
+