Logging

Another logging variation for your consideration that builds off of the ideas of [miguelperez](https://github.com/miguelperez/docusign_rest/commit/b80d490d70e1ad1f6a6774ed7bddbb1c8ce40711).

This is an attempt to standardize the log format to something that, hopefully, DocuSign will find acceptable for all & doesn't require making sense of the request & response objects.

Basic usage:
````
envelope = connection.create_envelope_from_document(doc)
connection.previous_call_log.each {|line| logger.debug line }
```

While I initially wanted to simply pass the log back in a method call's resulting hash, his approach seemed like the best alternative, as not all methods return a hash.