Regular Expression support for feed resource

While browsing through atom-hopper and abdera source code, I found out the existing code has was pretty close to exposing the functionality for creating dynamic feeds. For my requirements, I wanted to create feed named namespace/feed/<some_dynamic_identifier> without changing configuration in 'atom-server.cfg'. For implementing this I tried configuring feed reference as "/feed/[0-9]*", thats where I figured out making a small change in method getAnsweringAdapter of  "org.atomhopper.abdera.WorkspaceHandler" would be able to complete the functionality.

This change now allows, feeds to be created/updated/retrieved on the fly without having to enter the feed name explicitly in the configuration file.

I think this is quite a generic change, will add value to the current implementation, and many people in community will be interested in this. I found a TODO in the WorkspaceConfigProcessor as "sanitize configured workspace and feed resource paths for regex insertion". Not sure if the original author thought of in the same lines, but I think his intent can also be achieved this way.

Let me know if I have to follow a different procedure for this Pull Request.