Add Travel Acceleration and change the M204 options

Added option to set Travel Acceleration (non printing moves).
The M204 options was a non sense (S for printing moves and T for retract
moves).

It has been changed to:
P = Printing moves
R = Retract only (no X, Y, Z) moves
T = Travel (non
printing) moves

I will add this info o G-Code wiki in reprap.org. I also advise to put
this info in Marlin next version changelog.

Marlin/language_en.h

@@ -233,6 +233,9 @@
 #ifndef MSG_A_RETRACT
 #define MSG_A_RETRACT                       "A-retract"
 #endif
+#ifndef MSG_A_TRAVEL
+#define MSG_A_TRAVEL                        "A-travel"
+#endif
 #ifndef MSG_XSTEPS
 #define MSG_XSTEPS                          "Xsteps/mm"
 #endif