Add utilities for CRC32 checksumming

This pull request adds various utilities for calculating CRC32-based checksums.  See pull request #201 for an explanation of the ideas behind the checksum approach.  This includes a small test utility to calculate a CRC32-based checksum for an arbitrary file in a CVMFS cache.

The idea here is this will serve as the base of a lightweight fsck utility and, long-term, runtime verification of data files.