EasyID3 support for comment, url, and originalartist tags

[TravisCardwell]

I often use comments (COMM frames) and URLs (WXXX frames) in ID3v2.3 tags. I use EasyID3.RegisterKey to add support for these tags in my software, and I wonder if there is interest in adding support upstream. I implemented my proposed changes but have not created a pull request because I do not know if such changes are desired.

• I added a RegisterCommentKey class method that registers a comment key. The language and description can be specified as arguments. Note that I use a default language of \0\0\0 for compatibility with EasyTag, but I can change it to XXX for consistency with other tags, if desired. The method is called using the defaults, but users can call the method (again) with different values when desired.
• I added a RegisterUrlKey class method that registers a url key. The description can be specified as an argument. The method is called using the default, but users can call the method (again) with a different value when desired.
• I added a originalartist key (TOPE frame) to the mapping of keys registered using RegisterTextKey. In my experience, it is more commonly used than some other tags in the list.

If such changes are desired, I am happy to make any suggested changes and create a pull request.