Add thread-safe mock sink for testing (#68)

A common error in test code is to use a racy Store or Sink implementation and the existing MockSink cannot be used in a thread-safe manner by consumers of this library.  This PR adds a thread-safe Sink implementation that can be used with the existing Store code.

Additionally, the Sink has helper methods to make testing easier.

README.md

@@ -11,7 +11,7 @@ go get github.com/lyft/gostats
 ## Building & Testing
 
 ```sh
-make install 
+make install
 make test
 ```
 
@@ -22,3 +22,59 @@ In order to start using `gostats`, import it into your project with:
 ```go
 import "github.com/lyft/gostats"
 ```
+
+
+## Mocking
+
+A thread-safe mock sink is provided by the [gostats/mock](https://github.com/lyft/gostats/blob/mock-sink/mock/sink.go) package.  The mock sink also provides methods that are useful for testing (as demonstrated below).
+```go
+package mock_test
+
+import (
+	"testing"
+
+	"github.com/lyft/gostats"
+	"github.com/lyft/gostats/mock"
+)
+
+type Config struct {
+	Stats stats.Store
+}
+
+func TestMockExample(t *testing.T) {
+	sink := mock.NewSink()
+	conf := Config{
+		Stats: stats.NewStore(sink, false),
+	}
+	conf.Stats.NewCounter("name").Inc()
+	conf.Stats.Flush()
+	sink.AssertCounterEquals(t, "name", 1)
+}
+```
+
+If you do not need to assert on the contents of the sink the below example can be used to quickly create a thread-safe `stats.Scope`:
+```go
+package config
+
+import (
+	"github.com/lyft/gostats"
+	"github.com/lyft/gostats/mock"
+)
+
+type Config struct {
+	Stats stats.Store
+}
+
+func NewConfig() *Config {
+	return &Config{
+		Stats: stats.NewDefaultStore(),
+	}
+}
+
+func NewMockConfig() *Config {
+	sink := mock.NewSink()
+	return &Config{
+		Stats: stats.NewStore(sink, false),
+	}
+}
+```