Add prometheus_client service output module, update prometheus client

- Adds a client implementation using the prometheus go_client library
  that exposes metrics.

- Adds a new type of output "ServiceOutput" which follows inline with
  the "ServicePlugin", adding a Stop and Start method for the service

This change also requires the newer prometheus/client_golang code, so
the prometheus plugin needed to be changed.

Added the following to Godep:
    - bitbucket.org/ww/goautoneg (in github.com/common/expfmt/encode.go)
    - prometheus/common/expfmt (in plugins/prometheus.go)
    - github.com/prometheus/common/model (in plugins/prometheus.go)
    - github.com/prometheus/procfs (in github.com/client_golang/prometheus)
    - github.com/beorn7/perks/quantile (in github.com/client_golang/prometheus)

X-Github-Meta: closes #306

plugins/aerospike/aerospike.go

@@ -250,7 +250,8 @@ func get(key []byte, host string) (map[string]string, error) {
 func readAerospikeStats(stats map[string]string, acc plugins.Accumulator, host, namespace string) {
 	for key, value := range stats {
 		tags := map[string]string{
-			"host": host,
+			"host":      host,
+			"namespace": "_service",
 		}
 
 		if namespace != "" {