Added include/libreset/hash.h

This adds the include/libreset/hash.h file for the export of the hash
type used by the library.
Added documentation note about handling the rs_hash type
Updated README.md file
use size_t instead of uint64_t for rs_hash type
Merge pull request #24 from matthiasbeyer/add-hash_util

Add hash.h hash utility
Merge pull request #30 from matthiasbeyer/update-readme

Updated README.md file