conditionally include SD.h or SdFat.h depending on whether SDFAT is defined

SdFat.h should not be #include-d in TMRpcm.h and TMRpcm.cpp if SdFat is not being used. I've added some conditional directives to only include it if SDFAT has been #defined (and only include SD.h if it hasn't).

Prior to Arduino 1.5.7, this wasn't really a problem even if the SdFat library wasn't installed, because a missing header file only resulted in a compiler warning (normally hidden, visible only if you enable verbose compile output). Arduino's gcc was updated in 1.5.7, though, and now the missing header file will generate a fatal error and prevent the library and sketch from compiling; these changes fix the problem.