'as.tbl_cube' is missing a dispatch for matrix 

At the moment, as.tbl_cube() has no dispatch for matrix, which makes it inapplicable for matrices/two dimensional arrays:

```
m <- array(1, c(3, 3), list(x=1:3, y=1:3))
as.tbl_cube(m)
# Error in UseMethod("as.tbl_cube") :   
# no applicable method for 'as.tbl_cube' applied to an object of class "c('matrix', 'double', 'numeric')
```