Reorganize into flat structure, lock deps

* Rename app from bookshelf_wi to bookshelf
* Switch all config to use bookshelf app key
* Move src to top-level and adjust rel building accordingly
* Update Makefile for common template targets and dep locking
  goodness.
* Add rebar.config.lock
* Reformat reltool.config for automatic version bumping
* Add required ascii art for release generation
Return 403 if request URL is improperly (or not at all) signed

When an HTTP request hits bookshelf and is unsigned, return 403 not
500.
Add basic sanitizing of 500 error responses

* Add finish_request callbacks to the three wm resource modules. This
  could be improved in future by standardizing request state (Ctx) and
  using mixer to share the method from a common base module.

* Response body on 500 is text/plain with message "internal service
  error". This avoids spewing Erlang stacktrace which may contain
  sensitive information. A more expected response would be XML
  conforming to an S3 API error response, but for now this is an
  improvement.