Version spec revisions

Adding an EXTRA_INFO attribute to tor versions

Tor versions often contain information about the SCM commit they came from, for instance... 0.2.3.16-alpha-dev (git-8be6058d)

This isn't part of the spec, which in turn choked stem when I tried to parse those versions. Adding this in and better defining a couple other points...

• The STATUS_TAG should only contain non-whitespace. Otherwise... well, just about anything could be a 'valid' status tag.

• The spec says that status tags should be "compared lexically". The ASCII value of 'Z' is greater than 'A' so I guess this means that they're sorted in a reverse alphabetical order. This seems weird, but clarifying it.