Include humanized target state strings.

In the current /status implementation, we cannot divine what the
target's state is but rather get an integer constant for it.  This
commit, stringifies the constants.
Improve the base label representation in /status.

The base label representation under /status needs improvement to
enhance readability; namely, add sorting and make the label
representation concise.
Merge pull request #90 from prometheus/feature/scraping/named-target-state

Improve the Representations of /status