Add Test for Recursive Include Support

To guard against regression, and prove functionality, added a test and
associated example data to make sure nested `!include` directives work.

tests/data/examples/includes/all-the-properties.raml

@@ -0,0 +1,4 @@
+#%RAML 0.8
+
+external: !include properties.raml
+foo: !include foo-properties.raml

tests/data/examples/nested-includes.raml

@@ -0,0 +1,4 @@
+#%RAML 0.8
+title: GitHub API Demo - Includes
+version: v3
+include_one: !include includes/all-the-properties.raml

tests/test_loader.py

@@ -41,6 +41,26 @@ def test_load_file():
 
         assert dict_equal(raml, expected_data)
 
+def test_load_file_with_nested_includes():
+    raml_file = os.path.join(EXAMPLES + "nested-includes.raml")
+    with open(raml_file) as f:
+        raml = loader.RAMLLoader().load(f)
+
+        expected_data = {
+            'include_one': {
+                'external': {
+                    'propertyA': 'valueA',
+                    'propertyB': 'valueB'
+                },
+                'foo': {
+                    'foo': 'FooBar',
+                    'bar': 'BarBaz'
+                },
+            },
+            'title': 'GitHub API Demo - Includes',
+            'version': 'v3'
+        }
+
 
 def test_load_string():
     raml_str = ("""