Implement user migration

Extract users' first names, last names, email addresses and login names
from the XML.

First and last name extraction is best-effort, as JIRA has only a "name"
field.
Add dummy user to represent deleted JIRA users

To stop the migration failing in the case where a deleted JIRA user is
associated with an issue/comment/attachment, I added a 'ghost' user.
The ghost user is used to represent all deleted JIRA users.

This ghost user is only migrated to Redmine if it is needed.
It is locked (disabled), and it has the login name 'deleted-user'.
Merge pull request #2 from cb372/feature/migrate-users

Implement user migration