Refactored youtrack.coffee to support https connections.

The environment variable `HUBOT_YOUTRACK_SCHEME` was introduced which
defaults to "http". It is used to construct the youtrack urls using
`"#{scheme}://#{host}"`.

The `askYoutrack` and `login` methods were adapted and the option `host` was
replaced by `hostname` because according to the nodejs docs it should be
preferred.