Added Jira as external provider

Hi!

I created the option to use JIRA as external provider, while working for Meltwater. We use JIRA and sorcery internally. JIRA is a proprietary issue tracking product, developed by Atlassian.(http://www.atlassian.com/JIRA)

There are some configurations for JIRA that are different from other providers because JIRA is not a website like twitter that you can just go and create your account, most of companies has a domain like jira.company-name.com running somewhere. So to test I created this application: https://github.com/camilasan/jira-login-example

I tried to write tests, but it will only pass if you have JIRA running. 

I hope this is helpful. Let me know if you have any questions.