Fast CRC implementations ported from Intel's efforts

Provides fast CRC32C with PCLMULQDQ instructions in Golang

The white papers on CRC32C calculations with PCLMULQDQ instruction can be
  downloaded from:

http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/crc-iscsi-polynomial-crc32-instruction-paper.pdf
http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/fast-crc-computation-paper.pdf
Merge pull request #76 from harshavardhana/pr_out_fast_crc_implementations_ported_from_intel_s_efforts