Refactored to support https and base path.

2 new environment variables were introduced:

**HUBOT_YOUTRACK_BASE_PATH**

Includes the optional base path of the youtrack installation. If running
on tomcat this is usually "/youtrack".
The leading slash must be given and the trailing slash must be omitted.

**HUBOT_YOUTRACK_SCHEME**

Includes the connection scheme which can be either "http" or "https". It
defaults to "http" if not set.