Added append/success and append/error_unprocessable features

spec/test/features/control/append/error_unprocessable.feature

@@ -0,0 +1,45 @@
+Feature: Kapow! server reject responses with semantic errors.
+  Kapow! server will reject to create routes when
+  it receives a valid json document but not conforming
+  with the specification.
+
+  Scenario: Error by lack of mandatory fields.
+    If a request lacks of any of the mandatory fields
+    the server responds with an error indicating the
+    missing fields.
+
+    Given I have a running Kapow! server
+      When I append the route:
+        | entrypoint | command                    |
+        | /bin/sh -c | ls -la / \| response /body |
+      Then I get unprocessable entity as response code
+      And I get "Missing Mandatory Field" as response phrase
+      And I get the following entity as response body:
+        | missing_mandatory_fields |
+        | "url_pattern", "method" |
+
+  Scenario: Error by wrong route spec.
+    If a request contains an invalid expression in the
+    field url_pattern the server responds with an error.
+
+    Given I have a running Kapow! server
+      When I append the route:
+        | method | url_pattern  | entrypoint | command                    |
+        | GET    | +123--       | /bin/sh -c | ls -la / \| response /body |
+      Then I get unprocessable entity as response code
+      And I get "Invalid Route Spec" as response phrase
+      And I get an empty response body
+
+  Scenario: Error by wrong method value.
+    If a request contains an invalid value in the
+    field method the server responds with an error.
+
+    Given I have a running Kapow! server
+      When I append the route:
+        | method | url_pattern  | entrypoint | command                    |
+        | AVECES | +123--       | /bin/sh -c | ls -la / \| response /body |
+      Then I get unprocessable entity as response code
+      And I get "Invalid Data Type" as response phrase
+      And I get an empty response body
+
+...