add newmail_displayed, first_unread_message Cluaf

These functions combined with the indexhook method will allow to
allow to automatically jump to the first unread message when switching
from maildir do index. For some reason lumail will jump to the last
email now when switching from message to index mode
merge branch upstream/master
add index_hook function to lumail.lua

Implement the hook so that lumail jumps to the first unread/last message
of the selected folders. Also add some comments for the new code
fix typo
fix style
change first_new_message to jump_index_to_new

Changed the name of first_new_message to jump_index_to_new. The function
will now perform the jump operation instead of just returning the index
of the oldest new mail
move index hook to lumail.lua

remove the index hook from global.cc and move it to the index() function
in lumail.lua. This way the hook is configured completely as new
behavior in the lua script
move the complete logic into lumail.lua

All this can also be achieved with a few lines in lumail.lua. This is
also still fast when I open folders with several thousand emails at once
fix logic in lumail.lua/index()

Only change selected message when comming from maildir-mode
Merge remote-tracking branch 'upstream/master' into mutt_index_jumpstyle
key-bindings and use on_mode_change

The logic when switching form maildir to index is now called from the
on_mode_change hook. I also added keybindings to move to the next unread
mail in index or message mode.
Merge pull request #112 from kain88-de/mutt_index_jumpstyle

Cheers for your patience and dealing with my critique.