An M303 fix, some language fixes, and a CUSTOM_MENDEL_NAME option

I just implemented M303 before I realized it was already done, haha! But of course I didn't use interrupts so yours is better. But S0 should be silent, so I added code to handle that. The CUSTOM_MENDEL_NAME #define makes it easier to change the printer name as it appears in the LCD "Mendel Ready." message. Also did some English spelling and grammar corrections here and there.