Publish urls for Aishell model and Chinese language model.

Release librispeech model url.

fix a bug of external_memory.

Rewrite REAME.md for DS2 and update examples.

Add model configuration for machine translation with external memory.

fix travis.

Add clang-format hook

fix wrong path in prepare.sh of deep_speech2

fix missing import

Re-organize folder structure and hierarchy for DS2.

Reduce the config parsing codes (-360 Lines) for DS2 and make it looks cleaner.