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

Godeps/Godeps.json

@@ -5,6 +5,11 @@
 		"./..."
 	],
 	"Deps": [
+		{
+			"ImportPath": "bitbucket.org/ww/goautoneg",
+			"Comment": "null-5",
+			"Rev": "75cd24fc2f2c2a2088577d12123ddee5f54e0675"
+		},
 		{
 			"ImportPath": "git.eclipse.org/gitroot/paho/org.eclipse.paho.mqtt.golang.git",
 			"Comment": "v0.9.1-14-g546c47a",
@@ -24,6 +29,10 @@
 			"ImportPath": "github.com/armon/go-metrics",
 			"Rev": "b2d95e5291cdbc26997d1301a5e467ecbb240e25"
 		},
+		{
+			"ImportPath": "github.com/beorn7/perks/quantile",
+			"Rev": "b965b613227fddccbfffe13eae360ed3fa822f8d"
+		},
 		{
 			"ImportPath": "github.com/boltdb/bolt",
 			"Comment": "v1.0-117-g0f053fa",
@@ -127,25 +136,27 @@
 			"Rev": "ca53cad383cad2479bbba7f7a1a05797ec1386e4"
 		},
 		{
-			"ImportPath": "github.com/prometheus/client_golang/extraction",
-			"Comment": "0.7.0-22-gbbd006b",
-			"Rev": "bbd006bc5e64ea2c807381d50263be5f230b427d"
-		},
-		{
-			"ImportPath": "github.com/prometheus/client_golang/model",
-			"Comment": "0.7.0-22-gbbd006b",
-			"Rev": "bbd006bc5e64ea2c807381d50263be5f230b427d"
-		},
-		{
-			"ImportPath": "github.com/prometheus/client_golang/text",
-			"Comment": "0.7.0-22-gbbd006b",
-			"Rev": "bbd006bc5e64ea2c807381d50263be5f230b427d"
+			"ImportPath": "github.com/prometheus/client_golang/prometheus",
+			"Comment": "0.7.0-52-ge51041b",
+			"Rev": "e51041b3fa41cece0dca035740ba6411905be473"
 		},
 		{
 			"ImportPath": "github.com/prometheus/client_model/go",
 			"Comment": "model-0.0.2-12-gfa8ad6f",
 			"Rev": "fa8ad6fec33561be4280a8f0514318c79d7f6cb6"
 		},
+		{
+			"ImportPath": "github.com/prometheus/common/expfmt",
+			"Rev": "369ec0491ce7be15431bd4f23b7fa17308f94190"
+		},
+		{
+			"ImportPath": "github.com/prometheus/common/model",
+			"Rev": "369ec0491ce7be15431bd4f23b7fa17308f94190"
+		},
+		{
+			"ImportPath": "github.com/prometheus/procfs",
+			"Rev": "454a56f35412459b5e684fd5ec0f9211b94f002a"
+		},
 		{
 			"ImportPath": "github.com/samuel/go-zookeeper/zk",
 			"Rev": "5bb5cfc093ad18a28148c578f8632cfdb4d802e4"