add sentiment in __init__.py

python/paddle/v2/dataset/__init__.py

@@ -18,5 +18,8 @@ import imdb
 import cifar
 import movielens
 import conll05
+import sentiment
 
-__all__ = ['mnist', 'imikolov', 'imdb', 'cifar', 'movielens', 'conll05']
+__all__ = [
+    'mnist', 'imikolov', 'imdb', 'cifar', 'movielens', 'conll05', 'sentiment'
+]