Dvc

1) Add support for cached metrics `-m` to `dvc run`;

1) Fix pkb build in environments without gitpython;

1) Fix build issue on py2 on windows;

1) py2: fix unicode issue when working with os.environ.

1) Fix pyenv issue on travis;

1) Improve unicode support;
2) Status now returns non-zero code in `-q|--quiet` mode to reflect whether or not the pipeline is up-to-date;
3) Added support for `Content-Md5` as an `ETag` alternative in http(s) remote driver;
4) Unified coding style by using "black" tool;
5) Implemented `-R|--recursive` support for `dvc metrics show`; Kudos tdeboissiere ;
6) `dvc import` now supports `--resume` option to resume downloading if previous download failed;
7) Added support for `credentialpath`(aka `GOOGLE_APPLICATION_CREDENTIALS`) to gs remote driver;
8) `dvc checkout` now suggests `git checkout` just in case user mixed the two and tried to run `dvc checkout <branch>`; Kudos mhham ;
9) Fix bug in `dvc add` that would throw an error if you try to call it twice in a row on the same file;