BUG: None should be a valid colspec?
In https://github.com/pydata/pandas/blob/master/pandas/io/parsers.py, the **init** method for FixedWidthReaders checks whether the colspecs are valid.  Since None is a value slice index, should it be allowed as a colspec?

This would help me because I don't know the width of the last column until I start reading the file.

```
class FixedWidthReader(object):
"""
A reader of fixed-width lines.
"""
def __init__(self, f, colspecs, delimiter, comment):
    self.f = f
    self.buffer = None
    self.delimiter = '\r\n' + delimiter if delimiter else '\n\r\t '
    self.comment = comment
    if colspecs == 'infer':
        self.colspecs = self.detect_colspecs()
    else:
        self.colspecs = colspecs

    if not isinstance(self.colspecs, (tuple, list)):
        raise TypeError("column specifications must be a list or tuple, "
                        "input was a %r" % type(colspecs).__name__)

    for colspec in self.colspecs:
        if not (isinstance(colspec, (tuple, list)) and
                len(colspec) == 2 and
                isinstance(colspec[0], (int, np.integer)) and
                isinstance(colspec[1], (int, np.integer))):
            raise TypeError('Each column specification must be '
                            '2 element tuple or list of integers')
```
