Added string iterator in sam core.

See #2179

build/shared/revisions.txt

@@ -17,6 +17,7 @@ ARDUINO 1.6.9
 * Fixed Ctrl+Del: now deletes the word behind the cursor instead of the entire line. Thanks @avargas-nearsoft
 
 [core]
+* String class now supports iterators. Thanks @Chris--A
 * sam: Allow 3rd party boards that depend on SAM core to use their own
        USB vid/pid and manufacturer/product strings. Thanks @philmanofsky.
 * avr: Check at runtime if 32u4 boards are shipped with new bootloader; if so

hardware/arduino/sam/cores/arduino/WString.h

@@ -161,6 +161,8 @@ public:
 	void toCharArray(char *buf, unsigned int bufsize, unsigned int index=0) const
 		{getBytes((unsigned char *)buf, bufsize, index);}
 	const char * c_str() const { return buffer; }
+	const char* begin() { return c_str(); }
+	const char* end() { return c_str() + length(); }	
 
 	// search
 	int indexOf( char ch ) const;