Turn the proxy code into a library

Allow other go programs to easily import the snowflake proxy library and
start/stop a snowflake proxy.

proxy/lib/tokens_test.go

@@ -0,0 +1,28 @@
+package snowflake
+
+import (
+	"testing"
+
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+func TestTokens(t *testing.T) {
+	Convey("Tokens", t, func() {
+		tokens := newTokens(2)
+		So(tokens.count(), ShouldEqual, 0)
+		tokens.get()
+		So(tokens.count(), ShouldEqual, 1)
+		tokens.ret()
+		So(tokens.count(), ShouldEqual, 0)
+	})
+	Convey("Tokens capacity 0", t, func() {
+		tokens := newTokens(0)
+		So(tokens.count(), ShouldEqual, 0)
+		for i := 0; i < 20; i++ {
+			tokens.get()
+		}
+		So(tokens.count(), ShouldEqual, 20)
+		tokens.ret()
+		So(tokens.count(), ShouldEqual, 19)
+	})
+}