BUG: read_csv dtype inferrence is inconsistent for string columns with some integers
Large tsv files with columns that are mixed integers and strings are not consistently converted to the same data type.  Some of the integers are interpreted as integers and identical entries are interpreted as strings.  Transition seems to happen at power of 2 indices indicating something is being done in chunks but not for all chunks.

```
In [1]: import pandas as pd

In [2]: df = pd.DataFrame({'a':['1']*100000 + ['X']*100000 + ['1']*100000, 'b':['b']*300000})

In [3]: df.to_csv('test', sep='\t', index=False, na_rep='NA')

In [4]: df2 = pd.read_csv('test', sep='\t')

In [5]: print df2['a'].unique()
['1' 'X' 1]

In [6]: for a in df2['a'][262140:262150]:
   ...:         print repr(a)
   ...:
'1'
'1'
'1'
'1'
1
1
1
1
1
1
```
