Ability to import/export exercises and worksheets

[David Coles]

IVLE requires a way of importing and exporting worksheets and their associated exercises.

In particular it should be possible to easily import an external collection of worksheets (such as those from NLTK) or to facilitate distribution of content.

[Steven Bird (stevenbird)]

The initial priority is on import. This will permit us to author new worksheet content using restructured text markup.