add datasets import

python/paddle/v2/dataset/__init__.py

@@ -1,3 +1,7 @@
 import mnist
+import imikolov
+import imdb
+import cifar
+import movielens
 
-__all__ = ['mnist']
+__all__ = ['mnist', 'imikolov', 'imdb', 'cifar', 'movielens']