new property svn:tarballs for downloading
and extracting library tarballs.

* subversion/clients/cmdline/feedback.c: new message
  indicating the download of the tarball.
* subversion/clients/cmdline/main.c: help text for
  the new property.
* subversion/include/svn_config.h: two new entries
  for the config file specifying the external
  untar/unzip program and the arguments.
* subversion/include/svn_error_codes.h: errors which
  can occur during download of the tarball.
* subversion/include/svn_props.h: defines for the
  new property.
* subversion/include/svn_wc.h: new notify enum entry
  and new public function svn_wc_edited_tarballs().
* subversion/libsvn_client/checkout.c: make checkout
  also execute svn:tarballs.
* subversion/libsvn_client/switch.c: make switch
  also execute svn:tarballs.
* subversion/libsvn_client/update.c: make update
  also execute svn:tarballs.
* subversion/libsvn_client/client.h: new function
  svn_client__handle_tarballs().
* subversion/libsvn_client/tarballs.c: functions needed
  to download and decompress the tarballs.
* subversion/libsvn_subr/config_file.c: help info for
  the default config file.
* subversion/libsvn_wc/adm_crawler.c: while traversing
  the wc-tree also check for the svn:tarballs property.
* subversion/libsvn_wc/update_editor.c: fill in the 
  the traversal_info structure so the tarball functions
  know all svn:tarball properties later.
* subversion/libsvn_wc/wc.h: extend the svn_wc_traversal_info_t
  structure by the new property.