twee: Removed -a/--author option

It was used to specify the 'modifier' attribute for the tiddlers. This
is not used by Twine and as far as I know story formats don't use this
information either. Note this is unrelated to StoryAuthor.
Remove configurable TiddlyWiki author

Twee was the only piece of code that could override the default author
but that option was removed.
Removed 'modifier' attribute of tiddlers

It was not used anywhere.
Removed 'created' attribute of tiddlers

It was not used anywhere.
Removed 'modified' attribute of tiddlers

This used to contain the modification date, which was used to keep the
newest of two identically titled tiddlers. Instead of keeping track of
the modification date, we now just keep the latest added version. The
only situation when this could lead to different results is when
building a story with some of the tiddlers coming from an HTML file,
which is not a likely scenario.

Note that Tiddler.__cmp__ was implemented incorrectly, but since it was
used incorrectly as well, the two bugs cancelled each other out.
Omit 'tags' attribute for untagged passages

This produces slightly smaller and cleaner HTML.

Following the lead of TheMadExile, who did the same for TweeGo:
  http://twinery.org/forum/index.php/topic,1782.msg4657.html#msg4657
Merge pull request #162 from mthuurne/optional-tags

Remove various TiddlyWiki metadata from passages