Add SdPerf sample

Added support for building and linking in the mbed official SD card
based FAT filesystem code.

Also added the SdPerf sample which exercises it on a mbed-LPC1768.
Add date to Thanks section of README
fs: Silence GCC signed/unsigned warnings

The SDFileSystem class contained a few routines which compared a signed
integer loop index to an unsigned integer length/size.  I switched the
loop index to be uint32_t as well.