WString: add toDouble (#293)

Port of arduino/Arduino#5362

Author: sandeepmistry

Diff for: cores/arduino/WString.cpp

@@ -810,6 +810,11 @@ long String::toInt(void) const
 
 float String::toFloat(void) const
 {
-        if (buffer) return float(atof(buffer));
-        return 0;
+	return (float)toDouble();
+}
+
+double String::toDouble(void) const
+{
+	if (buffer) return atof(buffer);
+	return 0;
 }

Diff for: cores/arduino/WString.h

@@ -199,6 +199,7 @@ class String
 	// parsing/conversion
 	long toInt(void) const;
 	float toFloat(void) const;
+	double toDouble(void) const;
 	char * getCSpec(int base, bool issigned, bool islong);
 
 protected: