Fix parsing of quoted tags with spaces

Fixes gh-4
Implement tag mapping

Support a simple mapping to clean up and harmonize tags while migrating.
Clean up formatting and redundancy in log messages
Log error details on UnprocessableEntity error
Don't bork if assignee is not a collaborator

... instead just add a mention of the assignee to the body.

For general robustness to discrepancies between user mapping and current
collaborators, but also for the specific use case of test-migrating into
a fork with collaborators removed (to spare them the notification flood)
Log error details also when creating a milestone
Fetch all github milestones even if >30
Cache milestones fetched, not just those created

Matters when resuming a migration.
Merge pull request #5 from gthb/patch-1

Several fixes and minor improvements