Now HashedStreamReader will be used to calculate md5 in fobj_md5/hash_file.
Also:
* adds in `bytes_md5` that may be useful for testing.
* adds a hasher-registry with hashlib
* adds a `text=` kwarg to be able to specify to use dos2unix or not
(useful for testing)
* makes `localfs` the default filesystem when `fs` arg is not passed
(makes it easier to test in the REPL)