Update PHONY targets

Specifically, the `docs` target would fail to run in any repository that
had a `docs/` directory, like `riak-erlang-client`. This is because the
target was always considered up-to-date. You can test this by running
`make docs -n` with `riak-erlang-client` before and after this tools.mk
change. None of the other PHONY targets are likely to have
file/directory collisions, but it's better to add them just in case.
Merge pull request #5 from basho/bugfix/phony-targets

Update PHONY targets
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.