index: Move delete_repository over from the tags module

The rename from

  DELETE /v1/repositories/<namespace>/<path:repository>/

(which matches the spec [1,2]) to

  DELETE /v1/repositories/<namespace>/<path:repository>/tags

(not in the spec) happened in 1026a87 (renaming DELETE call on a
repos, 2013-06-26), but the motivation behind the rename is unclear.
I'm also not sure why a delete_repository function is in the
registry.tags module (which already has the expected delete_tag
method).

This commit restores the original URL, shifts the function to the
index module, triggers tag-deleted signals for each tag, removes the
remaining repository data en masse, and triggers the
repository-deleted signal.  It doesn't remove referenced images,
because those images might be required by other repositories.  We'd
want some sort of refcounting (number of repositories/tags requiring
this image) to decide when image-removal was allowed.

[1]: http://docs.docker.io/en/latest/api/registry_api/#delete--v1-repositories-%28namespace%29-%28repository%29-
[2]: http://docs.docker.io/en/latest/api/index_api/#delete--v1-repositories-%28namespace%29-%28repo_name%29-
index: Add '# nopep8' tags to long URL lines

This makes the lines even longer, but avoids tox failures:

  /.../registry/index.py:110:80: E501 line too long (116 > 79 characters)
  /.../registry/index.py:111:80: E501 line too long (112 > 79 characters)

Alternatively, we could ignore E501 entirely (with an entry in
tox.ini's flake8.ignore), but Sam Alba prefers ignoring just those
lines (and not all long lines) [1].

[1]: https://github.com/dotcloud/docker-registry/pull/247#issuecomment-36075734