chksum with MSVC Intrinsics

Add a version of chksum that uses MSVC intrinsics to compute CRC32 similar to the GCC version.

The intrinsics are documented here:
https://software.intel.com/sites/landingpage/IntrinsicsGuide/
http://msdn.microsoft.com/en-us/library/hskdteyh.aspx

The predefined MSVC macros are documented here:
http://msdn.microsoft.com/en-us/library/b0084kay.aspx