Added missing virtual destructors

Author: andreagilardoni

api/Printable.h

@@ -33,6 +33,7 @@ class Print;
 class Printable
 {
   public:
+    virtual ~Printable() {}
     virtual size_t printTo(Print& p) const = 0;
 };
 

api/Stream.h

@@ -62,6 +62,7 @@ class Stream : public Print
     virtual int peek() = 0;
 
     Stream() {_timeout=1000;}
+    virtual ~Stream() {}
 
 // parsing methods
 

api/String.h

@@ -81,7 +81,7 @@ class String
 	explicit String(unsigned long, unsigned char base=10);
 	explicit String(float, unsigned char decimalPlaces=2);
 	explicit String(double, unsigned char decimalPlaces=2);
-	~String(void);
+	virtual ~String(void);
 
 	// memory management
 	// return true on success, false on failure (in which case, the string