Add test for more complex default formatting...

and fix associated bug it exposed.
Make placehoder test's style construction more like the API
Remove defunct unit tests...

which were for the pre-refactor formatting code. I'm pretty sure I've covered
all the bases in the new code.
Test and fix block repr...

Yes, I'm going through and fixing up my unit test coverage :-)
Make use of Block.__iter__ in base...

Partly so that we cover the line in the unit tests, but that means we actually
use the darn thing!
Add tests covering updating of elements
Remove lingering misplaced lines!
Improve formatting API error message and cover with unit test
Add equality and repr tests for Placeholders
Add extra format to default format test...

which exercises the addition of a Placeholder to a PlaceholderGroup
Remove unnecessary logic in StringComponentSpec.populate
Added test of StringWithFormatting containment
Make Label inherity from Text, cover remaining implementation details with test