add support for package CHANGELOG

preliminary support is for a CHANGELOG.md, using all of the same logic
as for README.md, should one exist in a given package.
ditching semicolons
remove duplication in tab bars
`Readme.from_archive()` now takes an optional `name` (default value is `README`)

A CHANGELOG is just a specialization of a README file, as described
here:
http://en.wikipedia.org/wiki/README

This simplifies things greatly and allows us to avoid any duplication of
logic or class hierarchy.
remove unnecessary ChangelogProperty now that a changelog is a Readme

They share `data_type = Readme` so it makes sense to also reuse
`ReadmeProperty`; the `ChangelogProperty` was a relic of my earlier
changelist where I had introduced a `Changelog` data type / model.
adds test coverage of Readme's with the 'CHANGELOG' name pattern

Ran 104 tests in 14.988s
OK
Merge branch 'master' of https://github.com/dart-lang/pub-dartlang