nitdoc: fix github api plugin following spec changes

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: check github api upstream name correctness at plugin startup

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: exec without arguments terminates on exit(0)

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: change branch name box by link to github page

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: make titles in github plugin login box more explicit

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: make github 'Commit' button less scary with 'Commit...'

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>