Graph attributes

Send all sumd's attributes to graphite

With this PR, Riemann will send paths looking like this to graphite:

com.digital-science.www.http.metric
com.digital-science.www.http.task_time
com.digital-science.www.http.task_size

Rather than just one

com.digital-science.www.http

This assumes all attributes will have the name starting with `task_` - the way sumd sends them.

This will also change all non-sumd events to have `.metric` appended to their paths. This might look a bit ugly. Not sure how to get rid of that.

Tested on local vagrant setup.