[DEV] open new 1.6 development branch

This new branch is based on 1.5.0, which 1.6-dev0 is 100% equivalent to.
The README has been updated to mention that it is a development branch.

Released version 1.6-dev0 with the following main changes :
    - exact copy of 1.5.0
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 5ee2ae8..5f80674 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -2,7 +2,7 @@
                                 HAProxy
                           Configuration Manual
                          ----------------------
-                              version 1.5
+                              version 1.6
                              willy tarreau
                               2014/06/19