Add tests for xxhash
Implement xxhash32

1. adds a seed parameter to `digest()` to set the random seed used.
2. update documentation with the xxhash functions (also fix a few
spelling mistakes in existing documentation)
3. fixes a bug in the intial xxhash implementation (leading zeros were
not kept)
Merge pull request #4 from jimhester/xxHash

Add tests for xxhash