RippleSSLContext:  Add openssl wrappers
RIPD-108:  Use a self-signed certificate for peers

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.
[FOLD] RIPD-108:  Adjust cert validity

Per discussion with @JoelKatz.

Fold into 163370c789a0446f3e0f85856d7601482d322397.
[FOLD] RIPD-108:  Just throw std::runtime_error

Fold into 163370c789a0446f3e0f85856d7601482d322397.
[FOLD] RIPD-108:  Rename unique_ptr type alias

Rename it to custom_delete_unique_ptr to avoid confusion with the symbol
in std.

Fold into 163370c789a0446f3e0f85856d7601482d322397.