Ordered bytes in rawDigest output

Rusha#rawDigest used to return an Int32Array with the byte order
of the underlying platform and not sliced, which made the usage of the
underlying ArrayBuffer obtained by Int32Array#buffer hard (one had to
copy in another array using the proper endianness).

Changes a bit the API as rawDigest now reportedly returns an
ArrayBuffer instead of an Int32Array. This ArrayBuffer is sliced
at 20 bytes.

Added tests using the string 'abc' as an example.
Restore type compatibility of rawDigest

rawDigest again returns Int32Array, with a backing ArrayBuffer
being sliced to the hash length and containing bytes ordered
with the proper endianness.
Documentation mentions Int32Array in rawDigest
Merge pull request #11 from mathbruyen/rawdigestbytes

Ordered bytes in rawDigest output