In adafruit_sdcard.py, calculate_crc() re-create the CRC table each call, slowing all things down

[Anton-2]

calculate_crc() uses a computed crc_table to speed crc calculation up.

But this crc_table is calculated each time, for each command sent to the card, when it should be calculated once on module loading.

Moving this calculation at modules level helps making adafruit_sdcard.py faster.

[ladyada]

oh wow yeah! do you want to try submitting a PR for this, and testing it on your pygamer? then we could merge it in :)

[Anton-2]

Done in #23

[Anton-2]

Fixed in 21b7528, thanks for merging !