v0.2.8 Usability improvement release

Enhancements:

• Automatically calls datasets() when a workspace is created to validate the workspace id and auth token
• More informative error messages if AzureML responds with an error message
• Added images in the HTML help for workspace() to show where to find id and auth

Behind the scenes:

• Deals with differences in how the PROD and INT versions of the AzureML service returns results
• Refactored calls to always use expontential backoff - this means that all calls to AzureML retry several times if the service returns an error, and the elapsed time between calls grow exponentially.