Add the generate_solution function to the high level API

[ykazakov]

nbgrader --version

0.8.1

nbgrader has a command line option to generate solutions but there appears to be no respective function in the High-Level API.

Given that there is already a GenerateSolution converter, it should not be difficult to add this method: simply copy & paste the generate_assignment function and replace the GenerateAssignment app with the GenerateSolution app.

B.t.w., for some reason, the generate_assignment method is not mentioned in the documentation, even though it is used in the example.