allow to show the statusline on top

This is an experimental feature that allows to display the statusline in
the tabline. It might still be a bit rough, but seems to work so far.

Remaining problem:
- Mode changes are not immediately detected, only after moving the
  cursor

fixes #1388
closes #1867

autoload/airline/extensions/branch.vim

@@ -88,7 +88,8 @@ function! s:update_git_branch()
 
   let s:vcs_config['git'].branch = exists("*FugitiveHead") ?
         \ FugitiveHead(s:sha1size) : fugitive#head(s:sha1size)
-  if s:vcs_config['git'].branch is# 'master' && winwidth(0) < 81
+  if s:vcs_config['git'].branch is# 'master' &&
+        \ airline#util#winwidth() < 81
     " Shorten default a bit
     let s:vcs_config['git'].branch='mas'
   endif