Use IssueQuery class instead of Query class.

Need to call IssueQuery class to use the unique attr from Redmine 2.3.0.
Fix encoding error.
Fix coding style.
Use Query class to use the unique attr in Redmine < 2.3.0

Fix Issue #4
Modify Japanese locale file.
Update de.yml
Update ja.yml
Fix a bug which cannot be imported correctly when using underscore and number for user.login
Fix error message
store empty string as nil
result messages in report html instead of flash messages
Adds a param for substituting unkown users with the anonymous user.
Update ja.yml