Stcrestclient

Address an issue where a password parameter to a TestCenter command was printed in clear text if `debug_print` is enabled.

Added new examples of REST Bulk API and an example of an enhancement to `GetObjectsCommand` adding a `PropertyList` parameter which is more suitable in large scale scenarios. The enhancement to `GetObjectsCommand` is available in Spirent TestCenter version 5.51.

Addresses an issue with downloading some files via the stchttp.download() API when running the client with the STC REST server on Windows.

Add support for bulk operation APIs. Bulk operations are supported in Spirent TestCenter version 5.12 or later. The `has_bulk_ops` API call is available on `stchttp.StcHttp` to check if the bulk APIs are available.

Bulk API examples are available in the `examples` directory.

- Convert responses to str in python3
- Check for python2.7 or later
- Specify optional download location
- Update help and README
- Allow user to run tccsh and stcinfo as commands

- Adapter `new_session()` takes `existing_session` argument and recognizes `EXISTING_SESSION` environment variable to determine behavior when session already exists.
- Adapter `perform()` recognizes special commands whether or not they have "command" suffix.
- Does not try to use port 8888 on connection failure, which produces confusing error message.
- Provides additional examples passing parameters as kwargs.