pytables - optimized delete operation on a table
following code allows consecutive delete operations to use native tables methods and
consequently is much more effecient that individual row deletes. existing tests still pass.
#### replace HDFStore._delete_from_table with the following

```
def _delete_from_table(self, group, where = None):
    table = getattr(group, 'table')

    # create the selection
    s = hdf.Selection(table,where,table._v_attrs.index_kind)
    s.select_coords()

    # delete the rows in reverse order
    l  = list(s.values)
    ln = len(l)

    if ln:

        # if we can do a consecutive removal - do it!
        if l[0]+ln-1 == l[-1]:
            table.removeRows(start = l[0], stop = l[-1]+1)

        # one by one
        else:
            l.reverse()
            for c in l:
                table.removeRows(c)

        self.handle.flush()
    return ln
```
