Add "agile" API

[JamesMGreene]

JIRA has effectively deprecated the undocumented GreenHopper API and introduced a new Agile API to, uhh, sort-of take its place (although the GreenHopper API is still way more useful than its replacement, IMHO).

• JIRA Agile Cloud docs
• JIRA Agile Server docs

[satcheluniverse]

This issue should be fixed with #179 #180 #181 #182