* add train demo for imdb text classification task

* make inference library release data_feed dataset dataset_factory data_feed_factory

* add String Data Generator

* new feature of train demo: save model params

* New feature of train demo: set training config using gflags

* change code style for CI

* add readme and dataset for imdb demo trainer

* remove async executor and add data_feed.proto to the deps of train demo

test=develop

add cpp trainer lib and demo