Display a suitable message when no folders are selected.

  This fixes #34.
  Bind delete() to 'd'.

  This works in index/message-modes.
  Populate the References: header when replying.

  If the message we're replying to has a Message-ID then we'll
 add that to the References: header when sending the outgoing
 mail.

  This fixes #30.
  Reword minor sections of README.md.
  Demonstrate scheduling via on_idle().

  Here we update the message every second _AND_ resync mail every
 five minutes.
  Added CFile::executable()

  This tests if a given path is an executable (file).
  Added file-related primitives.

  Added:

    * executable
    * file_exists
    * is_directory

  These allow conditional things in the user config file.
  Return booleans from our file-related primitives.

  Previously we returned integers which was less useful.  Now this
 works as expected:

     if ( executable( "/usr/bin/imapsync" ) ) then
           msg("We have imapsync" )
     end
  New primitives: scroll_message_up / scroll_message_down.
  New global variable - offset of message to start display from.
  When displaying a single message start from the correct offset.

  This allows scrolling the message up/down.
  Cleanup for message scrolling.

  Space now pages-down a message.
  Tidy the default configuration file.

  Print a quotation on termination.
  Removed example functions.

  These are best kept online: http://lumail.org/examples/
  Removed more examples; added link to examples online.
  Reword the header.

  Added a link to the online-examples too.
  Changed the name of the folder-selection hook.

  We're now called on_folder_selection.
  Call our hook when folders are cleared - with a null-path.
Fix for Issue #35: remove static buf
  Avoid unsigned int + int comparisons.