In io.ascii, fall back to string if integers are too large

If I try and read the following file:

```
a                             b
12121312311248721894712984728 1122
```

using ``Table``, I get:

```
In [5]: t = Table.read('data', format='ascii')
ERROR: OverflowError: Python int too large to convert to C long [astropy.io.ascii.core]
---------------------------------------------------------------------------
OverflowError                             Traceback (most recent call last)
<ipython-input-5-bcb96ef3f221> in <module>()
----> 1 t = Table.read('data', format='ascii')

/Volumes/Raptor/Library/Python/3.3/lib/python/site-packages/astropy-0.4.dev7437-py3.3-macosx-10.8-x86_64.egg/astropy/table/table.py in read(cls, *args, **kwargs)
   1725         passed through to the underlying data reader (e.g. `~astropy.io.ascii.ui.read`).
   1726         """
-> 1727         return io_registry.read(cls, *args, **kwargs)
   1728 
   1729     def write(self, *args, **kwargs):

/Volumes/Raptor/Library/Python/3.3/lib/python/site-packages/astropy-0.4.dev7437-py3.3-macosx-10.8-x86_64.egg/astropy/io/registry.py in read(cls, *args, **kwargs)
    317 
    318         reader = get_reader(format, cls)
--> 319         table = reader(*args, **kwargs)
    320 
    321         if not isinstance(table, cls):

/Volumes/Raptor/Library/Python/3.3/lib/python/site-packages/astropy-0.4.dev7437-py3.3-macosx-10.8-x86_64.egg/astropy/io/ascii/connect.py in read_asciitable(filename, **kwargs)
     19 def read_asciitable(filename, **kwargs):
     20     from .ui import read
---> 21     return read(filename, **kwargs)
     22 
     23 io_registry.register_reader('ascii', Table, read_asciitable)

/Volumes/Raptor/Library/Python/3.3/lib/python/site-packages/astropy-0.4.dev7437-py3.3-macosx-10.8-x86_64.egg/astropy/io/ascii/ui.py in read(table, guess, **kwargs)
    154         guess = _GUESS
    155     if guess:
--> 156         dat = _guess(table, new_kwargs)
    157     else:
    158         reader = get_reader(**new_kwargs)

/Volumes/Raptor/Library/Python/3.3/lib/python/site-packages/astropy-0.4.dev7437-py3.3-macosx-10.8-x86_64.egg/astropy/io/ascii/ui.py in _guess(table, read_kwargs)
    196 
    197             reader = get_reader(**guess_kwargs)
--> 198             dat = reader.read(table)
    199 
    200             # When guessing require at least two columns

/Volumes/Raptor/Library/Python/3.3/lib/python/site-packages/astropy-0.4.dev7437-py3.3-macosx-10.8-x86_64.egg/astropy/io/ascii/core.py in read(self, table)
    851 
    852         self.data.masks(cols)
--> 853         table = self.outputter(cols, self.meta)
    854         self.cols = self.header.cols
    855 

/Volumes/Raptor/Library/Python/3.3/lib/python/site-packages/astropy-0.4.dev7437-py3.3-macosx-10.8-x86_64.egg/astropy/io/ascii/core.py in __call__(self, cols, meta)
    649 
    650     def __call__(self, cols, meta):
--> 651         self._convert_vals(cols)
    652 
    653         # If there are any values that were filled and tagged with a mask bit then this

/Volumes/Raptor/Library/Python/3.3/lib/python/site-packages/astropy-0.4.dev7437-py3.3-macosx-10.8-x86_64.egg/astropy/io/ascii/core.py in _convert_vals(self, cols)
    631                     if not issubclass(converter_type, col.type):
    632                         raise TypeError()
--> 633                     col.data = converter_func(col.str_vals)
    634                     col.type = converter_type
    635                 except (TypeError, ValueError):

/Volumes/Raptor/Library/Python/3.3/lib/python/site-packages/astropy-0.4.dev7437-py3.3-macosx-10.8-x86_64.egg/astropy/io/ascii/core.py in converter(vals)
    590 
    591     def converter(vals):
--> 592         return numpy.array(vals, numpy_type)
    593     return converter, converter_type
    594 

OverflowError: Python int too large to convert to C long
```

Maybe when the integers are too large, strings should be used instead? This is a simplified version from the issue described in http://stackoverflow.com/questions/22617428/overflowerror-python-int-too-large-to-convert-to-c-long-with-astropy-table.