New function `fits_get_rowsize'
New function `fits_create_ascii_tbl' and `fits_create_binary_tbl'

These two new functions wrap the same CFITSIO function
(`fits_create_tbl'). The reason why there are two implementations is
because the original function offered to create two different kinds of
HDUs: IMO, having this spelled in the function name leads to clearer
code.

Also, instead of passing the three arrays TTYPE, TFORM, and TUNIT to the
function, the Julia wrapper accepts just *one* array of `ColumnDef's:
this reduces the possibility of errors.