ErrataConnection: add raw kwargs to _get method
ErrataConnection: add raw kwargs to _get method

- added doctext to this method
- added support for 'raw' bool kwargs defaulting to False
- return the whole response object if raw set, otherwise
  return json() results like normal

This is to allow usage of _get() for retreiving other content than API
where the returned content is always json object.

User of this option is responsible for handling error checking and decoding

red-hat-storage/errata-tool,ktdreyer/errata-tool,red-hat-storage/errata-tool,ktdreyer/errata-tool