Add course homework answer (PaddlePaddle#583)

* Fix dataset doc

* fix dataset doc

* Change load_dataset design for loading local dataset

* Update softmax_with_cross_entropy to cross_entropy

* Fix community provided datasets and fix cross_entropy for elmo and dgu

* Add course homework answer

Update README

tag v2.0.0

fix deadlink (PaddlePaddle#423)

fix badge error