Add ability for jira-issue to ignore posts from certain users.

Here is a fix for https://github.com/github/hubot-scripts/issues/960

I introduced a new environment variable HUBOT_JIRA_IGNORE_USERS, which defaults to "jira|github". This means by default the script will ignore references to JIRA tickets by the GitHub and JIRA users. 

In our use-case, this is used to ignore the GitHub and JIRA Hipchat connectors, but I assume this is applicable to other chat systems (IRC, Campfire, etc) as well.