Stratocumulus

Fix `sync` command for local backend (PR 26 by yihming )

Feature improvement for cp command in local backend:

Automatically create parent folders if target is a directory. (PR 22 , 23 by joshua-gould )
Support wildcard *. (PR 24 , 25 by yihming )

* Add `-r` option back to `strato cp` command for copying local folders to AWS S3 bucket. (PR 18 by joshua-gould )
* `strato exists` command on AWS S3 folders now uses **s3fs** instead of **boto3** package, in order to resolve the conflicts with **pandas** `read_csv` function which are brought by later version of **botocore** package. (PR 17 by joshua-gould ; PR 19 by yihming )

Drop `--backend` option and infer backend from the target URI. (Thanks to joshua-gould in PR 13 )

Bug fix on AWS backend copy command with wildcards:
* Make such commands work for `subprocess.check_call` in the case when source paths are local. (PR 11 )
* Fix a bug for the case when source paths are relative with wildcards at top level, i.e. `*.txt`. (PR 12 )

* Bug fix for `cp` command with wildcards for AWS backend.