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

outputs/registry.go

@@ -5,13 +5,35 @@ import (
 )
 
 type Output interface {
+	// Connect to the Output
 	Connect() error
+	// Close any connections to the Output
 	Close() error
+	// Description returns a one-sentence description on the Output
 	Description() string
+	// SampleConfig returns the default configuration of the Output
 	SampleConfig() string
+	// Write takes in group of points to be written to the Output
 	Write(points []*client.Point) error
 }
 
+type ServiceOutput interface {
+	// Connect to the Output
+	Connect() error
+	// Close any connections to the Output
+	Close() error
+	// Description returns a one-sentence description on the Output
+	Description() string
+	// SampleConfig returns the default configuration of the Output
+	SampleConfig() string
+	// Write takes in group of points to be written to the Output
+	Write(points []*client.Point) error
+	// Start the "service" that will provide an Output
+	Start() error
+	// Stop the "service" that will provide an Output
+	Stop()
+}
+
 type Creator func() Output
 
 var Outputs = map[string]Creator{}