Specify the encoding when reading files.

In Ruby 1.9, we have to specify the encoding explicitly because the
magic comment doesn't change the default external encoding (US-ASCII).
Add test_comment.rb.
Fix Comment.delete.

There is a bug that any user can delete any comment.
Fix resource leaks.
Use do and end for a multi-line block.
Add tests about a access control list of comments.
Merge pull request #8 from chiro/fix-encoding-issue

Fix encoding issue