Add bin/bup symlink so bin/ can be added to PATH

Signed-off-by: Rob Browning <rlb@defaultvalue.org>

diff --git a/bin/bup b/bin/bup
new file mode 120000 (symlink)
index 0000000..1969131
--- /dev/null
+++ b/bin/bup
@@ -0,0 +1 @@
+../lib/cmd/bup
\ No newline at end of file