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.
Merge pull request #4 from chocoby/fix_unique_attr

Use IssueQuery class instead of Query class.
Merge pull request #5 from chocoby/fix_encoding_error

Fix encoding error.
Merge pull request #6 from chocoby/fix_coding_style

Fix coding style.