correct gnus faces

  * gnus-group-news-low-empty -> was completely invisible solarized-bg;
    use solarized-comment instead.

  * remove -face postfix. gnus-*-face variables don't take effect (for
    unclear reason they take effect on the manual initialization of the
    theme with load-theme).

  * for consistency all gnus-*-high-* variables are now boldified
    versions of gnus-*-normal-* versions.
Merge pull request #74 from vitoshka/master

correct gnus faces