Move JSON code below /codec

To prevent package import cycles, the interface
`EncodeDecoder` is also moved into `codec` where it fits naturally.

Fixes #19

codec/json/json_test.go

@@ -0,0 +1,11 @@
+package json
+
+import (
+	"testing"
+
+	"github.com/asdine/storm/codec"
+)
+
+func TestJSON(t *testing.T) {
+	codec.RountripTester(t, Codec)
+}