refactor: Migrate add verb into its own DSL file

The `case object add` was previously located into the
`PartialUpdateObject.scala` file. However, we need to use it for the
upcoming AB Test implementation.

This commit moves the entire `case object add` into its own
`AddDsl.scala` file, like what we already have for existing common verb
such as `get` (`GetDsl.scala`), `list` (`ListDsl.scala`), etc.

src/main/scala/algolia/AlgoliaDsl.scala

@@ -37,6 +37,7 @@ import scala.util.matching.Regex
 trait AlgoliaDsl
     extends Object //Just to have all trait DSL ordered `with`
     with KeyDefinitionDsl
+    with AddDsl
     with BatchDsl
     with BrowseDsl
     with ClearDsl

src/main/scala/algolia/dsl/AddDsl.scala

@@ -0,0 +1,45 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2016 Algolia
+ * http://www.algolia.com/
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package algolia.dsl
+
+import algolia.definitions.{Add, AddKeyDefinition, PartialUpdateObjectOperationDefinition}
+import algolia.objects.ApiKey
+import org.json4s.Formats
+
+trait AddDsl {
+
+  implicit val formats: Formats
+
+  case object add {
+
+    def value(value: String): PartialUpdateObjectOperationDefinition =
+      PartialUpdateObjectOperationDefinition(Add, value = Some(value))
+
+    def key(key: ApiKey) = AddKeyDefinition(key)
+
+  }
+
+}

src/main/scala/algolia/dsl/PartialUpdateObjectDsl.scala

@@ -51,15 +51,6 @@ trait PartialUpdateObjectDsl {
 
   }
 
-  case object add {
-
-    def value(value: String): PartialUpdateObjectOperationDefinition =
-      PartialUpdateObjectOperationDefinition(Add, value = Some(value))
-
-    def key(key: ApiKey) = AddKeyDefinition(key)
-
-  }
-
   case object addUnique {
 
     def value(value: String): PartialUpdateObjectOperationDefinition =