Metabasic

Enhancements
- Adding `raw_query(query, export_format)` to retrieve responses in one of the supported export formats for `/api/dataset/{export_format}` (`csv`, `json`, `xslx`)

Bug fix
- `get_dataframe` uses `raw_query` with `export_format="csv"` to guarantee that the result set has a column ordering equal to the ordering specified by the query statement

Enhancements
- Adding use of api/dataset/json to circumvent metabase row limit of 2000

Enhancements
- Adds get_dataframe utility method, returning a pandas DataFrame
- Updates query method to return the raw 'data' of the API resposne

Enhancements
- Update authenticate & select_database methods to allow chaining

Enhancements
- Add better handling of unconfigured/unauthenticated clients

Enhancements
- Initial version