TODO

For hmac:
* Support hashing using keys larger than the base block size of the hash
* Trap more errors to make sure things die loudly.

For s3:
* Do some sort of "real" XML parsing instead of the ugly hackjobs currently
  used.
* More xml encoding escapes. 
* Better dependency checking.  I rely on alot of GNUisms, but there just
  does not seem to be a really portable way of finding a file size. :(
  This would involve matching revisions of utilities as well as their 
  presence.
* x-amz-whatever headers.  Currently, signature generation supports including
  them, and curl_headers can be used to portably generate them, but there
  is no method of actually introducing them into the HTML bitstream.
* Support for just passing the S3 url instead of having the tool construct one.
  I don't need it, and I think that writing a URI parser for a short shell
  script is Too Much Work, but someone probably thinks it is interesting.
* HEAD variants of s3_list and s3_bucket.  Because sometimes all we really
  want are the headers.

For both:
* Figure out the earliest version of bash that offers all the bashisms we 
  rely on, and code the scripts to die loudly of we don't have it.