Migration does not continue if it was interrupted

[rata]

Hi!

First, thanks a lot for this project! We are using it to migrate from trac to github and it seems to work quite well! :-)

Although, during a migration github returned a 502 and there is no way to continue the migration. And seems we didn't find any way to delete the issues (as you say in the README), we have to delete and re-create the repository.

Right now I'm running it again in the re-created repository and hope it will finish this time. Just in case, when it throw a 502 this was the output:

Traceback (most recent call last):
  File "./trac-tickets-to-gh.py", line 154, in <module>
    gh_issue = github.issues(data=issue)
  File "./github.py", line 49, in issues
    return self.access(path, query=query, data=data)
  File "./github.py", line 38, in access
    raise RuntimeError("Error on url=%s e=%s" % (url, e))
RuntimeError: Error on url=https://api.github.com/repos/<org>/<repo>/issues e=HTTP Error 502: Bad Gateway

(I've just changed the path to the files and the organization in the URL)

When I try to re-run with an incomplete migration, this is the output:

Traceback (most recent call last):
  File "./trac-tickets-to-gh.py", line 124, in <module>
    gh_milestone = github.milestones(data=milestone)
  File "./github.py", line 70, in milestones
    return self.access('milestones', query=query, data=data)
  File "./github.py", line 38, in access
    raise RuntimeError("Error on url=%s e=%s" % (url, e))
RuntimeError: Error on url=https://api.github.com/repos/<org>/<repo>/milestones e=HTTP Error 422: Unprocessable Entity

It will be nice if a migration could be resumed, although I don't know if this change is worthless. For big trac this might be useful quite useful, though :-)

Thanks a lot,
Rodrigo