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/IXSocketTLSOptions.cpp

@@ -58,6 +58,10 @@ namespace ix
         return caFile == kTLSCAFileUseSystemDefaults;
     }
 
+    bool SocketTLSOptions::isUsingInMemoryCAs() const {
+        return caFile.find("-----BEGIN CERTIFICATE-----") != std::string::npos;
+    }
+
     bool SocketTLSOptions::isPeerVerifyDisabled() const
     {
         return caFile == kTLSCAFileDisableVerify;