De-EXPERIMENTALIZE the sqlite backend and IDLE folders

Both have been in a stable release and we have never gotten negative
feedback about them. I have been using sqlite exclusively for some time
and people seem actively to use the IDLE folders without major problems.

This patch removes the scary: "THIS IS EXPERIMENTAL" warning, that I
know has kept some people from using it. Do note that the plaintext
backend is still the default even with this patch.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap.conf

@@ -201,8 +201,7 @@ remoterepository = RemoteExample
 # state in plain text files. On Repositories with large numbers of
 # mails, the performance might not be optimal, as we write out the
 # complete file for each change.  Another new backend 'sqlite' is
-# available which stores the status in sqlite databases. BE AWARE THIS
-# IS EXPERIMENTAL STUFF.
+# available which stores the status in sqlite databases.
 #
 # If you switch the backend, you may want to delete the old cache
 # directory in ~/.offlineimap/Account-<account>/LocalStatus manually
@@ -379,8 +378,8 @@ remoteuser = username
 #     holdconnectionopen - to be true
 #     keepalive - to be 29 minutes unless you specify otherwise
 #
-# This feature isn't complete and may well have problems. BE AWARE THIS
-# IS EXPERIMENTAL STUFF.  See the manual for more details.
+# This feature isn't complete and may well have problems. See the manual
+# for more details.
 #
 # This option should return a Python list. For example
 #