M300 bug, CUSTOM_MENDEL_NAME, SD Dir Depth, grammar

- I implemented M300 before I realized it was already done, oops! But of course I didn't use interrupts so DDRBoxman's is better. But S0 should be silent, so I fixed that.

- #define CUSTOM_MENDEL_NAME makes it easier to change the printer name as it appears in the LCD "Mendel Ready." message.

- Added code to support up to 10 levels of depth in the SD folder hierarchy.

- English spelling and grammar corrections here and there.