Added Sexp property to get 2d arrays

- Its possible to Make an Sexp with a 2d array of doubles or ints, but
there's no way get the 2d array back.  Its flattened into a list
- Consequently, reading a matrix from R required manual work to
represent the matrix in .net as a 2d array
- Resuscitated the test project to test this new feature
- Added xunit as testing framework