RSS feed support

You can now subscribe to your twister postboard via RSS feed reader. Incoming direct messages are synchronized too. Feed URL is /rss on your twister client, for example http://127.0.0.1:28332/rss

Parameters: If you have more than one username, you can specify which one you want with the “account” parameter. If you want to change number of items, use the “max” parameter, default is 20. Format is the same as in any other HTTP URL. For example:

http://127.0.0.1:28332/rss?account=johndoe&max=10

Implements feature request #161