Add Jabber Agent

Hey, really enjoying playing with huginn.

In the past I most often used IM for quick alerts. Getting Jabber integration was big for me on this project. I tried to keep the change minimal. This one can connect to a specified jabber server and will send a message which parses the payload into a mustache string. Been using it this past week and it's been doing pretty well.

I tried to follow style as best I could, feel free to request any changes you'd like @cantino