Add an example acceptance test

test/acceptance/execute_capability.feature

+Feature: Users can submit capability requests
+
+  The most basic usage of the Workspaces system is for a user to submit
+  some kind of capability request and have something interesting happen.
+
+  Scenario: User runs a capability
+    Given the system is online
+    When a user submits the null capability
+    Then the null program is executed
+
+  Scenario: User runs the download capability
+    Given the system is online
+    When a user submits the download capability
+    Then files appear in the delivery area
+
+
+Feature: Stretch goals for capability requests
+
+  These are goals we may or may not achieve this week
+
+  Scenario: User runs the download capability
+    Given the system is online
+    When a user submits the download capability
+    Then files appear in the delivery area
+    And  The download complete notification is sent