WIP upload docs target.

The goal of this is to upload generated edocs to S3 so they can be
viewed by the code reviewer.
Fix typo
Add trailing slash to s3 URL
Print docs URL upon uploading

* Print the new docs URL. This assumes 'website hosting' is turned on
  for the bucket.
* Redirect stdout of s3cmd to /dev/null. It's noisy.
Change the upload-docs target to bail when any env var is unset.
Seems parens in test args are not portable, or too picky in syntax.
Merge pull request #6 from basho/feature/edoc

Upload edoc output to S3 when building