add keepUnsentMessages config option (default true)

Ext.ux.WebSocket has a keepUnsentMessages config property, which will buffer messages until the connection is open. This commit makes this config also available for Ext.ux.data.proxy.WebSocket.

On startup data sometimes doesn't load with autoLoad: true. With keepUnsentMessages: true this is no longer a problem.
Merge pull request #13 from richard-vd/master

add keepUnsentMessages config option (default true)