src/riemann.proto: Import the protocol buffer for Riemann 0.2.0.
write_riemann plugin: Use the "Attributes" of an Event rather than abuse the "Tags".

Riemann 0.2.0 provides "Attributes", which are key-value pairs. Since
the plugin previously basically abused the tag system for adding this
kind of data, switch to the new and shiny thing :)
write_riemann plugin: Reorder the functions to get rid or prototypes.