Standardize Import/Export calls through Vistrails API

[okennedy]

Notebook Import/Export is currently handled through the server API. This should be rewritten to go through the Vistrails API

• Avoiding race conditions from going through the filesystem for state
• Supporting import/export for other 'versions' of the API (e.g., if we want to migrate to using Mimir's metadata store as a backend storage API)