Supporting more attributes. Inc. font face and size.

Shadow now works. Striketrhough works etc.
Some clean up.

No longer depending on CoreText fonts.

Some misc. warnings cleared up & dead code removed.

Tag handlers now take a context parameter. Can use it to inspect state of
attributed string so far - can use it to get current colour and font size
etc.

UILabel now has a default markup value transformer.
Some more refactoring, tidy up and hiding of unnecessary public code.

Also we don't need to subclass NSValueTransformer.