Add OpenSslEngine

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.

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