Prototype of `fits_read_col' changed

The function does no longer return an array, but instead writes the
entries in the `data' parameters. This makes the prototype of this
function more similar to `fits_read_pix'.

Also, the input parameters are declared to be 'Integer' rather than
Int64, in order to allow greater flexibility.
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.
New function `fits_write_col'
New functions `fits_get_col_repeat', `fits_insert_rows', and `fits_delete_rows'
`fits_read_col' now reads strings correctly
`fits_write_col' now works with strings.
Small fixes to `fits_read_col'

Unused variable removed. The code now checks for ASCIIString types as
well as plain String types.