Added missing virtual destructors

Author: andreagilardoni

api/Print.h

@@ -42,6 +42,8 @@ class Print
     void setWriteError(int err = 1) { write_error = err; }
   public:
     Print() : write_error(0) {}
+    virtual ~Print() {}
+
 
     int getWriteError() { return write_error; }
     void clearWriteError() { setWriteError(0); }

api/Printable.h

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

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