ssl: Add Client Side Certificate Auth

[lian]

Add Client Side Certificate Auth feature and handling to puma's MiniSSL. Also exposes SSL errors to puma/apps.

compatibility notes: MRI only

shell example:

puma -b 'ssl://127.0.0.1:9292?key=path_to_key&cert=path_to_cert&ca=path_to_ca&verify_mode=force_peer'

code example: (examples/client_side_ssl)

app = proc {|env| p env['puma.peercert']; [200, {}, ["hey"]] }

events = SSLEvents.new($stdout, $stderr)
server = Puma::Server.new(app, events)

admin_context             = Puma::MiniSSL::Context.new
admin_context.key         = KEY_PATH
admin_context.cert        = CERT_PATH
admin_context.ca          = CA_CERT_PATH
admin_context.verify_mode = Puma::MiniSSL::VERIFY_PEER | Puma::MiniSSL::VERIFY_FAIL_IF_NO_PEER_CERT

server.add_ssl_listener("0.0.0.0", ADMIN_PORT, admin_context)
server.min_threads = MIN_THREADS
server.max_threads = MAX_THREADS
server.persistent_timeout = IDLE_TIMEOUT
server.run.join

additional credits: Andy Alness andy.alness@gmail.com