Merge branch 'next'

* next:
  doc,README: new help mapping
  plugin/magit.vim: handle empty commit (without staged file) and empty commit message (fix #79)
  autoload/magit/utils.vim: fix strip_array out of bound
  plugin/magit.vim: add magit#get_staged_files() and magit#get_unstaged_files() functions
  plugin/magit.vim: change inline help toggling mapping from 'h' to '?' (fix #80)