archive_xlog now creates the local directory if missing
Do not initialize target_timestamp in restore if not required

Just to make things clean. The target_timestamp must be computed only if
target_date is provided on the commandline. There is any bug here
because target_date is always checked, thus preventing the use of an
uninitialized target_timestamp.
remove the pitr_mgr symlink
update de upgrade doc with major changes for 1.7
Configuration of compression tools for base backups

When using the tar storage method, let the user configure the tools to
compress the files. Add new configuration parameters:
BACKUP_COMPRESS_BIN, BACKUP_COMPRESS_SUFFIX and BACKUP_UNCOMPRESS_BIN,
as well as command line switched for backup_pitr and restore_pitr.