Add an OpenSslEngine and the universal API for enabling SSL

Motivation:

Some users already use an SSLEngine implementation in finagle-native. It
wraps OpenSSL to get higher SSL performance.  However, to take advantage
of it, finagle-native must be compiled manually, and it means we cannot
pull it in as a dependency and thus we cannot test our SslHandler
against the OpenSSL-based SSLEngine.  For an instance, we had #2216.

Because the construction procedures of JDK SSLEngine and OpenSslEngine
are very different from each other, we also need to provide a universal
way to enable SSL in a Netty application.

Modifications:

- Pull netty-tcnative in as an optional dependency.
  http://netty.io/wiki/forked-tomcat-native.html
- Backport NativeLibraryLoader from 4.0
- Move OpenSSL-based SSLEngine implementation into our code base.
  - Copied from finagle-native; originally written by @jpinner et al.
  - Overall cleanup by @trustin.
- Run all SslHandler tests with both default SSLEngine and OpenSslEngine
- Add a unified API for creating an SSL context
  - SslContext allows you to create a new SSLEngine or a new SslHandler
    with your PKCS#8 key and X.509 certificate chain.
  - Add JdkSslContext and its subclasses
  - Add OpenSslServerContext
- Add ApplicationProtocolSelector to ensure the future support for NPN
  (NextProtoNego) and ALPN (Application Layer Protocol Negotiation) on
  the client-side.
- Add SimpleTrustManagerFactory to help a user write a
  TrustManagerFactory easily, which should be useful for those who need
  to write an alternative verification mechanism. For example, we can
  use it to implement an unsafe TrustManagerFactory that accepts
  self-signed certificates for testing purposes.
- Add InsecureTrustManagerFactory and FingerprintTrustManager for quick
  and dirty testing
- Add SelfSignedCertificate class which generates a self-signed X.509
  certificate very easily.
- Update all our examples to use SslContext.newClient/ServerContext()
- SslHandler now logs the chosen cipher suite when handshake is
  finished.

Result:

- Cleaner unified API for configuring an SSL client and an SSL server
  regardless of its internal implementation.
- When native libraries are available, OpenSSL-based SSLEngine
  implementation is selected automatically to take advantage of its
  performance benefit.
- Examples take advantage of this modification and thus are cleaner.