Applies labels to github issues

We look for labels in the format of "##name" in the comments of newly
opened issues and existing issues.

A label / list of labels in a comment will replace all existing labels.

Pull requests are automatically tagged "is: pull request". The comment
when opening a pull request is also checked for labels (but in practice,
probably not something that's going to be used that often)

Valid labels are:

* ##from: suggestions/support
* ##is: bug/feature
* ##effort: average/average/higher
* ##severity: minor/major/critical

Labels not on that list won't be applied.

Fixes dreamwidth/dw-free#701
Add status: untriaged for new pull requests/issues

Also make sure that we always add "type: pull request/issue", but
without wiping any existing labels.
Indicate we want to use SSL
Use urls from the event payload instead of hardcoding urls

* so it works on both dw-free/dw-nonfree (also so that there's no need
  to change when testing vs in prod)

* send Authorization header instead of sending the token via the URL

* tweak the %GITHUB hash format, because we already have a %GITHUB hash
  -- so put the token and shared secret into %GITHUB{api}
Merge pull request #63 from afuna/github-interface

Hooks to interact with github