rdiff: new tempdir config option, useful when rdiff-backup fills up /tmp

i was getting IOError: [Errno 28] No space left on device because my /tmp was to small. and i don't want to set/change the global variables TMPDIR, TEMP, and TMP :)

With this Merge Request we can pass the --tempdir option to rdiff-backup and everything is back in business.

thanks for your awesome work.

examples/example.rdiff

@@ -66,6 +66,12 @@
 ##
 ## Default:
 # output_as_info = no
+#
+
+# temporary directory used by rdiff-backup
+# (default = /tmp or /usr/tmp, depending on the system)
+#tempdir = /tmp
+
 
 ######################################################
 ## source section