Freeze the fields of a tiddler

Thus enforcing the immutability of tiddler objects
Add support for ordering tags by the 'list-before' and 'list-after' fields

See the discussion here:

https://groups.google.com/d/topic/tiddlywikidev/AXDJEjcAphs/discussion
Exclude attributes starting "on" on HTML elements

Because:

* It doesn't work well with TW5's refresh mechanism, which relies on
being able to regenerate any portion of the DOM as required; this
frequently causes inline handlers to be re-executed at unexpected times
(see
http://tiddlywiki.com/static/TiddlyWiki%2520for%2520Developers.html)
* It mixes TW5 version-specific JavaScript with user content
* In multiuser environments there is a security risk to importing or
viewing tiddlers you didn't author if they can have JavaScript in them
signed CLA
Update to latest version of normalize.css and fix tab styling

Tab styling was inconsistent on Firefox due to some line-height issues.
Update link to tags docs
Merge pull request #484 from TheDiveO/master

signed CLA
added descriptions for list-before, list-after
Left off previous commit
Fix setTextReference()

It wasn’t working for non-existent tiddlers. Also added support for
setting indexed properties of a data tiddler.
Extend tag manager to edit icons assigned to tags

Add a dropdown for choosing available images for a tag.

As part of this change we need to tag tiddlers that contain images
typed as wikitext (these are inline SVG icons) so that we can display
them in the dropdown.
Merge pull request #485 from TheDiveO/master

added descriptions for list-before, list-after
Tweaks to list-before and list-after field descriptions
Default colour tweaks
Add support for tw-home message