Add categories and XModuleDescriptors for all module types that are used at the top level of a course

fix breakage in problem checker caused by LoncapaProblem taking a file object in its constructor instead of a filename

customresponses can't tell us what their answer is (there can be a
wide range of answers -- it's verified by a small program). So here
we simply ignore those fields we can't generate answers for when
doing tests against problems.

Update documentation for functions inside common/xmodule

Move courseware tests into the common/xmodule library