Add cntk backend (#103)

* add cntk backend

* reorder imports

keras_contrib/backend/__init__.py

@@ -12,3 +12,5 @@ if K.backend() == 'theano':
     from .theano_backend import *
 elif K.backend() == 'tensorflow':
     from .tensorflow_backend import *
+elif K.backend() == 'cntk':
+	from .cntk_backend import *

keras_contrib/backend/cntk_backend.py

@@ -0,0 +1,2 @@
+from keras.backend import cntk_backend as KCN
+import cntk as C