correct gnus faces


* gnus-group-news-low-empty -> was completely invisible solarized-bg;
    use solarized-comment instead.

before: 
![gnus_invisible](https://f.cloud.github.com/assets/1363467/906933/95f39962-fcf8-11e2-9dbc-d4afaa985c19.png)

after:
![gnus_corrected](https://f.cloud.github.com/assets/1363467/906934/b5e0b214-fcf8-11e2-951e-5d49ba052754.png)


  * 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 this reason gnus ticked and high mails are colored by gnus default ugly pink    instead of the solarized bold orange.

  * for consistency all gnus-*-high-* variables are now boldified
    versions of gnus-*-normal-* versions.


before:
![gnus_ugly_pink](https://f.cloud.github.com/assets/1363467/906938/de396530-fcf8-11e2-8909-d2206aeb863c.png)

after:

![gnus_pink_corrected](https://f.cloud.github.com/assets/1363467/906942/3403756e-fcf9-11e2-8d63-914ac511c0a5.png)