Use the issue export feature instead of polling the API

[Erik van Zijst]

We now have a proper issue export feature that produces a zip file that contains all issue information for a repo, including attachments and issue logs.

It's better to use that and to write a converter for the json file that is in the archive.

Info can be found here: https://confluence.atlassian.com/display/BITBUCKET/Export+or+Import+Issue+Data