NNI v0.5.1 release

Release 0.5.1 - 1/31/2018

Improvements

• Making log directory configurable
• Support different levels of logs, making it easier for debugging

Documentation

• Reorganized documentation & New Homepage Released: https://nni.readthedocs.io/en/latest/
• Chinese users are able to learn NNI with the translated Chinese doc:
  https://github.com/microsoft/nni/blob/master/README_zh_CN.md
  Dear Contributors: We'd love to provide more language translations, contribute to NNI with more languages =)

Bug Fixes and Other Changes

• Fix the bug of installation in python virtualenv, and refactor the installation logic
• Fix the bug of HDFS access failure on PAI mode after PAI is upgraded.
• Fix the bug that sometimes in-place flushed stdout makes experiment crash