Add development code for SD card library using the SPI protocol.

Features working so far include: card initialisation, commands, block reads, register reads (CID and CSD) and CRC7.

Requires an external SPI implementation with a number of basic functions, see extern definitions in sd_spi.h.
Initial work to support the CSD register, which contains various card specific parameters such as size, capabilities and timing.

Test project can now read card size in bytes for SDSC and SDHC/SDXC.

Added brief doxygen comments at top of file.
Initial checkin of block read for data, which works with the test application but needs more work.

Clarify why extra clocks are required in sd_stop() by referring to the standard.
Update readme file, add simple script to convert Proteus ARES ZIP files to a format suitable for OSHPark.
Rename .MD to .md through an intermediate filename to get around Windows/SourceTree stupidity^Wcase sensitivity.
Delete old file
Hopefully rename file to lowercase extension
Update readme with some links to popular code
Correct file filter, fixes issue #1 with ZIP selection on Mac platform.