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.h

@@ -36,6 +36,8 @@ namespace ix
         bool hasCertAndKey() const;
 
         bool isUsingSystemDefaults() const;
+        
+        bool isUsingInMemoryCAs() const;
 
         bool isPeerVerifyDisabled() const;