Add basic image io and remove python C-API refs from numpy_returns.cpp (#1258)

* Fixed reference count issue

* Fixed refcount issue in Python dlib.jitter_image and dlib.get_face_chips

* Consolidation of https://github.com/davisking/dlib/pull/1249

* Fixed build issue

* Fixed: Paths in a pytest file should be relative to dlib root

* Skip numpy return tests for Python 2.7 or if Numpy is not installed

* Enabled numpy returns tests on Python 2.7 using cPickle.dumps

tools/python/test/utils.py

+import pkgutil
+import sys
+
+def save_pickled_compatible(obj_to_pickle, file_name):
+    '''
+        Save an object to the specified file in a backward compatible
+        way for Pybind objects. See:
+        http://pybind11.readthedocs.io/en/stable/advanced/classes.html#pickling-support
+        and https://github.com/pybind/pybind11/issues/271
+    '''
+    try:
+        import cPickle as pickle  # Use cPickle on Python 2.7
+    except ImportError:
+        import pickle
+    data = pickle.dumps(obj_to_pickle, 2)
+    with open(file_name, "wb") as handle:
+        handle.write(data)
+
+def load_pickled_compatible(file_name):
+    '''
+        Loads a pickled object from the specified file
+    '''
+    try:
+        import cPickle as pickle  # Use cPickle on Python 2.7
+    except ImportError:
+        import pickle
+    
+    with open(file_name, "rb") as handle:
+        data = handle.read()
+        if not is_python3():
+            return pickle.loads(data)
+        else:
+            return pickle.loads(data, encoding="bytes")
+
+def is_numpy_installed():
+    '''
+        Returns True if Numpy is installed otherwise False
+    '''
+    if pkgutil.find_loader("numpy"):
+        return True
+    else:
+        return False
+
+def is_python3():
+    '''
+        Returns True if using Python 3 or above, otherwise False
+    '''
+    return sys.version_info >= (3, 0)