Decode Content-MD5 with base64, not hex

Also fix crypto fun names, though I haven't yet
seen a situation where that function is called.
More discussion of the issues can be seen
[here](https://github.com/basho/webmachine/issues/116).
Merge pull request #117 from basho/wrd-base64-content-md5

Decode Content-MD5 with base64, not hex