Make ApiClient more pluggable for Java

- Rename ApiInvoker to ApiClient
- Make ApiClient pluggable by allowing setting the ApiClient field of
  API classes
- Introduce a Configuration class, containing the default
  ApiClient (which is also customizable)
- Move basePath from API class to ApiClient
- Change static methods in ApiClient to instance level

modules/swagger-codegen/src/main/resources/Java/Configuration.mustache

@@ -0,0 +1,13 @@
+package {{invokerPackage}};
+
+public class Configuration {
+  private static ApiClient defaultApiClient = new ApiClient();
+
+  public static ApiClient getDefaultApiClient() {
+    return defaultApiClient;
+  }
+
+  public static void setDefaultApiClient(ApiClient apiClient) {
+    defaultApiClient = apiClient;
+  }
+}