Create Ripple SSL contexts using std::make_shared.
RippleSSLContext:  Add openssl wrappers
Use a self-signed certificate for peers (RIPD-108):
Generate a new RSA key pair and a self-signed X.509v3 certificate to use
with SSL connections to rippled peers.  New credentials are created each
startup.
Enable std::array overloads for boost::asio on clang:
* Remove Boost config option from beast config.
* Define from compiler, or let Boost figure out itself.
Make streambuf buffers_type iterators default constructible
Add shared_ptr<boost::asio::ssl::context> to ssl_bundle:
This gives the ssl_bundle shared ownership of the underlying ssl context
so that ownership of the bundle may be transferred to other classes without
introduce lifetime issues.
Refactor Overlay and add [overlay] config section (experimental):
These changes prepare Overlay for the Universal Port and Hub and Spoke
features.

* Add [overlay configuration section:
The [overlay] section uses the new BasicConfig interface that
supports key-value pairs in the section. Some exposition is added to the
example cfg file. The new settings for overlay are related to the Hub and
Spoke feature which is currently in development. Production servers should
not set these configuration options, they are clearly marked experimental
in the example cfg file.

Other changes:
* Use _MSC_VER to detect Visual Studio
* Use ssl_bundle in Overlay::Peer
* Use shared_ptr to SSL context in Overlay:
* Removed undocumented PEER_SSL_CIPHER_LIST configuration setting
* Add Section::name: The Section object now stores its name for better diagnostic messages.
Refactor HTTP::Server to support Universal Port:
These changes are necessary to support the Universal port feature. Synopsis:

* Persist HTTP peer io_service::work lifetime:
This simplification eliminates any potential for bugs caused by incorrect
lifetime management of the io_service::work object.

* Restructure Door to prevent data races, and handle clean exit:
The Server, Door, Door::detector, and Peer objects work together to
correctly implement graceful stop and destructors that block until
all child objects have been destroyed.

Cleanups:
* De-pimpl HTTP::Server
* Rename ServerImpl data members
* Tidy up HTTP::Port interface