Add a benchmark for MarshalEvent.
Add a benchmark for MarshalEvent.

  $ go test -run=NONE -bench="MarshalEvent" -cpuprofile=cpu.out
  PASS
  BenchmarkMarshalEvent	  100000	     15742 ns/op
  ok  	sourcegraph.com/sourcegraph/appdash	2.046s

joeshaw/appdash,joeshaw/appdash,bg451/appdash,bg451/appdash,joeshaw/appdash,bg451/appdash