Improve handling of issues that are detected to have already been migrated, toward implementation of an update-existing feature (to import updates to already migrated issues).
Nicer little container class for representing issue identifiers; mostly just avoids having to constantly string-format issues.
Break out the code that migrates new issues into a separate function, just because import_issues was getting a bit long...
Created an actual main() function so that the code executed in the body of the module doesn't leak variables into the module namespace, among other reasons
Moved most of the code responsible for creating new issues into a separate function.  Determining new milestones and labels is handled later, outside this function--this is more reorganization toward supporting updating already migrated issues.