Let users map Github usernames to JIRA usernames via configuration.

[brandizzi]

We are relying on the JIRA search for mapping the Github username into a JIRA username. It sometimes fails. For example, if "John Doe" is "jdoe" at Github and "john.doe" at JIRA, it will not work.

With this pull request, we can add a new configuration to the JIRA plugin, where we map the GitHub name to a JIRA name.

@henvic