Implement API for adding custom roots via a string (#178)

* Implement API for adding custom roots via a string. SocketTLSOptions API design needs work, but the IXSocketOpenSSL implementation feels good to me.

* Improve API design for specifying roots from memory.

* Add in-memory root CAs mbedtls implementation.

* Fix bug in newer versions of OpenSSL with in-memory certificate handling.

ixwebsocket/IXSocketOpenSSL.h

@@ -40,6 +40,7 @@ namespace ix
         void openSSLInitialize();
         std::string getSSLError(int ret);
         SSL_CTX* openSSLCreateContext(std::string& errMsg);
+        bool openSSLAddCARootsFromString(const std::string roots);
         bool openSSLClientHandshake(const std::string& hostname,
                                     std::string& errMsg,
                                     const CancellationRequest& isCancellationRequested);