TextBlock style update 

Hi all,

This one passes tests and I ran through a few samples and it seems to behave as expected!

I did make a bunch of changes that I'll comment on inline which should be reviewed. I'm pretty sure this can be improved in a couple of places but I'm not sure how.

@jasonrclark I'm not sure how this should mix with #832. Based on what I see I think they should play nicely.
The specs call on a generic `TextBlock` which means they don't call the subclass (`Para` `Banner` etc.). The subclasses have the `:font` style key so that means only in the specs is it possible for `style[:font]` to come back `nil`.